Howdy! What I’m looking for… Well, there should be at least one gameplay thing your hack does so well, it’s notable for that alone, whether it’s a completely new idea or an iteration on something that came before.
Personally I loved how Fire Emblem Fates Conquest Lunatic made Unit-building an interesting long-term question weighed against the cost of investing your limited resources into that unit.
Who do you invest EXP and boosters into and why? How do you pair units to share Classes and Skills? What do you do with someone like Jacob who can reclass and access promoted skills so early in the game and pass them down to his kid, and to yours if he’s marrying Corrin? What do you do with someone like Laslow who’s surrounded by superior sword options but can give anyone Sol and give Ninja skills to his kid? This unit’s great if you give him this skill, but that requires pairing him with him, but he’d rather be with her to get her skill, and she’s great in her base class but she’d rather be reclassed so she can be promoted and give a better skill to her kid. Everything’s connected.
A boring answer like “pair the two Berserkers together so their kid has more Strength” would be fine in Awakening, but here it comes with a cost. Opportunity cost. Do that and you’re not giving the parents or children the best Skills outside their usual class options like Life And Death, Quick Draw, Sol, Replicate, or the Breaker skills. Make them better at what they were meant to do or reclass them into something completely different. It’s got more going on than Awakening where you make overpowered kids who pair up and beat the game for you. Sure, Fates still has overpowered options like Maid Camilla and Sol Maid Camilla and Vantage Vengeance Life And Death Ophelia, but people are still making videos analyzing the units because there’s a lot here worth thinking about.
Also, you have something tangible to measure a unit’s greatness and the opportunity cost of training them: The ease of capturing and recruiting an enemy to do that. Laslow’s personal skill screams “Rally user” but you can capture a better Rallyman. You can even capture impossible things like Ninjas with the Beastman skill for wounding enemies post-combat AND the Ninja skill that does the same. Guess those particular ninjas are secretly furries.
EXP and Gold is limited, so you need to think about what to spend it on and where to get more. Personally I like pairing Elise and Arthur ASAP for the Gold from their kid’s mission.
I think Engage and 3 Houses completely missed the mark in that area. There’s no real opportunity cost when it comes to making everyone the best possible version of what they can be. Yeah, your Dark Mage can spend time as a Mage to learn Fiendish Blow. Bernadetta doesn’t even have a hard time picking up Vengeance and Stealth to become untargetable by enemies as long as another ally is nearby and capable of killing anything ever in one shot. You don’t need to weaken your combat unit by giving him a crappy low-rank weapon to gain weapon EXP, just train with it outside of combat during menus. Half the damn game is menus. No, 75% of the game is menus. Menus and the accursed Monastery. Breaking the game is so easy, it makes you wonder why the plot makes a bigger deal out of Crests than Engage makes out of the Emblem rings. Don’t get me started on Engage where you can just save up and spend your Primogems to get OP. “Hey man how do I make this unit good?” “Buy Tiki and inherit the skill that buffs growths”.
FE9/10 are probably my favorites overall (loved switching armies in 10!), but 3H is a great contender. Fates has gotten my most replays in recent years simply due to the variety with routes and kid units (if only the story wasn’t 
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