So I wanted to do a little research to see what people would enjoy or hate in a game so I could optimize my hack or maybe get new ideas for it.
(Before anyone says don’t take the community’s statement as facts, I’m aware. I just want to get an idea and maybe some inspiration. Sorry if this paragraph sounded rude)
Generally speaking, I like it when a FE game or hack has a stat or weapon type that you’re lacking for a lot of the early game or even all game long, this makes the gameplay stand out and forces the player to develop unique strategies, especially if they heavily relied on that weapon type or stat.
It could be as short a time as chapter 1 of the Binding Blade not having a healer or be as long as lacking sizable HP bars the entirety of Conquest unless you have a berserker that specializes in having high HP.
I like it when things are tight in a game. This applies across all facets of the game. I prefer not to have a game plot spiral out of control into killing god unless the starting premise of the game was to kill god. Core mechanics that are novel should be introduced early and used with some consistency. You should have a grasp of the creator’s psychology when it comes to map design relatively early on. Things like that.
Edge.
I hate edgelords, especially when they’re the main character.
FE10
Post must be at least 5 characters
I like avatar characters. You dont need them to be the main MC but they at least need to have supports like an average unit and maybe appear every now and then. And with them only female supports for paired endings.
A thing I have noticed is that many hacks are stingy with exp so most of the time by the end of the game you aren’t likely to cap any character so would like a better exp share that let’s me max at least the main party I use (Like say of you only give 12 promotion seals, it would be nice if there was enough exp to cap those 12).
not gonna offend someone but some rom hackers that puts way too much skills on every character Idk it just kills off how i enjoyed gba fe I only like it when they use only one simple skill or just literally nothing but learn them from a class promotion(i like how they did in fe8 but needs more 2nd tier class skills tbh). Also overly sized maps with tons of stuff and side objectives in it that’s a complete slog to play and even frustrating to look at. Even a bs map from kaga’s thracia is alot better and fun than this (if they’re just a small 20 below tiled map) but maps that are overly huge with tons of crap the designer likes to put and it’s in the early game is just a big turn off for me. Also also I hated it when rom hackers edit weapons from vanilla it’s either a rebalance or to for difficulty purposes like for ex. give slim weapons insanely high crit for idk what reason. I also dislike the use of “starter weak weapons”(bronze weapons) since they give no purpose like why not just get straight into iron.
This is what is politely referred to as skill bloat or a skillsfest. I along with some others are on the more radical spectrum of absolutely abhorring it. In the opinion that skills should have very clear purposes and be easily recognizable. In particular to keep more brainless play, which in turn helps the flow of every map as well as general pacing of the whole game.
The rest is more down to execution.
I’m a weirdo and I love big map fights. And, to go along with it, I love politics that come with it. Deity Device scratches that itch greatly for me, for example, chapter 10 of ‘disk 1’ so to say. It’s awesome to see how much your characters grow experience wise and more often than not the plot ending of that map is game-changing. It’s just cool 
Oddly enough, in an about-face, I really dislike large casts. I want to get emotionally evolved with the whole cast and I really dislike it when I’m finding myself picking up 4 units a map. Most aren’t that bad but sometimes it really feels like it. I dislike having a great team working together for 3 chapters and getting that bonding feeling from the story when -WHAM- you’ve found yourself with 12 more characters and now you have to choose between 20 characters which 8 to use. And when you’ve finally made the hard choice… it’s not even -WHAM- anymore, it’s just -wham- now you have 40 to decide to choose. When you’re like me and emotionally indecisive, it breaks my care for the characters/story and I more often than not stop giving a wham.
Yeah I’ll have to agree with the whole characters thing. Sometimes it’s just waaaay too much.
I’m the type of fan who’s willing to sacrifice design “tightness” in favor of just letting the player do what they want, so I appreciate when hacks do have difficulty settings and don’t force you to play FE6 hard mode with five whole viable units if you don’t want to. I know it’s a really small thing, but having a mode for each type of player makes games more accessible and more fun to more, different people, even if the difference is just autolevels.
Fog bad, and my excitement tanks everytime it appears in a map, and I hate that 3H brought it back after the 3ds games lacked fog (unless you count the weird gimmicks in ch 7/10 of rev)
The same goes with me, I’m not very good at Fire Emblem so when I’m forced into too much “hardcore” difficulty, I tends to just left the game because I either softlocked myself by loosing too many units (or not taking the few that are above the rest) or got bored after having too many resets on a chapter. 
Fog is cool if the enemy is also subject to vision restrictions and the player has options to increase or reduce vision range.
unfortunately I doubt think the GBAFE engine has enough meat to do dynamic checks for whether the player unit is in fog for the enemy even with asm unless one wants to stress that piddly memory limit.
As for what I like and dislike in FE games… unit raising shouldn’t be the main draw. I like having the option to break the game over my knee but it shouldn’t be a slog or otherwise annoying to play without doing unit raising or optimization - outside of on a per-map strategic deployment choice basis. And speaking of deployment choice, making maps and mechanics that stress choosing which units to deploy is really neat! Obviously depending on player habits it’s not viable to force this choice late game, but that’s where the much-appreciated developer decision to cut out most of the “gotta try hard” meat in late game maps in favor of the opportunity for satisfying execution of all the tools the player has gained, plus heavy gameplay-story integration. Radiant Dawn endgame was actually alright although I’m still salty about Ike becoming excessively relevant yet not being the protagonist or necessary to resolve the conflict aside from some asspull about ragnell
My favorite part of FE3H was Gambits. AoE spells and active skills are usually my most hated SRPG mechanics, but the way Gambits were implemented was satisfying, and as a bonus, maps didn’t even need to be designed around them; they’re mostly tools for getting out of screwy situations or creating advantageous ones. And vice versa ofc
Having Demonic Beast Gambit mechanics in a GBAFE romhack is on my top 3 wishlist lol
I like small amounts of skill to make a class unique. don’t really like personal skills though. also to many skills tend to bother me since it means more reading time between attacks and decisions which I don’t like to do that much
I like when there is a big cast. With a smaller cast, you lose only one of your units and you reset because you are not sure to have more than 3 units with the same class in your party
With a larger cast, you don’t have this problem. If one of you favorite die of course you reset but otherwise no need to
I think that this is a good mechanic for players like me who sometimes make strategic mistakes.
For the emotional attachment , you can always add 3 support for each characters but they will be less characters involved in the story. However, remember that the lord is like à commander and the others units are soldiers
What i personally like is subjective. Since every hack has different expectations or requirements I tend to mainly think “am i having fun” without really thinking about why.
But what i don’t like, Hoo boy can i certainly explain that.
I don’t like early game flyers that die if a fe7 soldier looks at them with a cocked eyebrow.
I don’t like when every unit worth using is on a horse.
I don’t like starting off with Two lance users an archer and a priest and fight brigands for 4 chapters and end up recruiting a brigand and a cleric just to fight more axe users for two more maps.
Same as the last one but with sword users instead of lances.
I’m not exactly keen on crit=threatening or a boss only being hard/threatening because their weapon gives them +20 Def Res and Speed.
I don’t like being expected to have played Genealogy or Thracia to know about particular maps gimmick.
I don’t care for the lack of interesting weapon ideas that have come about as of late.
I’m not fond of being expected to use a dancer who can’t contribute more then just the refresh.
Honestly i have more but those are just the immediate ones that pop into my head.
i hate how people treat armor knights in general (making them too good or bad)
they’re my fav type of class purely out of bias, honestly speaking lol. but it feels like people either go too far in making them good or they’re the typcial bad one. whether its fe6 with their garbage stats and depressing mov, or project ember where generals are just the best infantry class with no exception
i feel they are typically best when they have 5 mov promoted and unpromoted, with lance, axe, and bow on general (swords are awesome too and add extra firepower but obviously not needed lol), making using an armor not the worst since it’s a normal infantry, and its large amount of weapon diversity on promo is balanced with its below average mov
I like the way “dark lord and the maiden of light” made them, dividing the base class into (lance,axe) armors and (swrd,bow) armors really made them unique, on promo to general they get all the physical weapons unlocked which as you say helps to make up for poor move. One of my fav units in the hack is the grey bow wielding knight you get and I carried her to the last map, She rocks and can actually double things as general.
If possible, I’d like some unit customization. It’s probably a bit hard to do so in hacks other than making a “nameable unit”. Branching promotion is good to, and probably could be considered unit customization I guess?
What I hate? Perhaps a very hard map that relies too much on luck and trial & error I guess?