What if the game invisibly counted every bad roll for stat gains, and upon deciding you’re not at a high enough power level for where you are in the story, the RNG becomes rigged in your favour by increasing degrees of obviousness?
There might even be a RNG-based chance for Fallout 1/2 style Random Encounters to happen with every crossed tile on the World Map, a natural evolution of those Merchants and Risen encounters in Awakening. Games balanced around the expectation that you’ll never die can prevent softlocks with last-minute additions of “Revenge Values” that make your units able to pick up the slack of a dead unit even in games like 3H.
Of course such the RNG mercy system should ignore optional power gains the player had control over. If you got unlucky and your Lord is awful and has no chance of beating the final boss for the true ending, time for RNG to take pity on you and buff that lord, maybe spring a random encounter on you for a sweet bonus. But if you played slow and missed your chances to slay enemy thieves for their expensive gems before they flee, protect or talk to villages for a sweet reward, and make the sacrifice of fielding a Thief instead of a stronger fighter and take the risk of sending him after enemies to rob them blind, that’s on the player, not the luck system.
FE11 does something a little like this, where characters have dynamic growth rates. Essentially, every time a character misses a growth from a level, the game adds 10% of that growth to the stat’s growth. So, if a unit misses a 40% growth once, it becomes 44%. This can stack, so the longer you go without getting a stat, the more likely it is to happen. After missing four times, the 40% growth becomes 58~59%. Take note that this growth boost resets after a stat actually levels, once the 40% growth levels the unit up, it’s back to 40%. This functions as a gentle push to your units to keep them close to average, along the lines of what you said for RNG mercy.
Random encounters have their advantages no matter how loudly others voice their disdain for it but in a system like Fire Emblem I feel random encounters would just take too much time.
As for the RNG mercy system I believe it should be entirely optional since in my opinion this system completely defeats the purpose of random level ups, trying to make sure that player’s units meet their averages ensures that most people’s units are going to be the same which is good from a game design standpoint but horrible from the standpoint of it being a Fire Emblem game, random level-ups are part of its identity and being RNG-blessed or cursed makes a person’s experience stand out all the more.
If you’re worried the player’s lord isn’t strong enough to beat the final boss you should rethink the final boss entirely. There are more than enough tools in most if not all Fire Emblems to ensure that even if your lord doesn’t gain any stats at all that they can still beat the final boss if they even have to in the first place (imo it’s just not very fun to be forced to beat the final boss with one unit in particular).
While random encounters are something i don’t much enjoy, the mercy system could be useful in a more controlled enviroment. You could make it affect item drops, for example if the player has most their units strength screwed in a defeat boss map whose boss is a general, the system could add a few armor-effective weapons or magic swords to the drops/chests, it could also check for the party’s average stats and drop stat boosters for those stats that are far below the curve (i.e adding an extra speedwing when most enemies in a chapter would double all your units). That purpose, to keep the rng from screwing the player TOO harshly is why these games tend to have a constant supply of new units, with stats generally on the level that the game demands at that point, but should the devs continue on the non-permadeath friendly design of three houses a mercy system could solve a lot of that game’s issues.
For the random encounters, Gaiden and Echoes already had reinforcements on the world map that were spawned from RNG. They only particularly interesting thing from them is when they ambush you, causing your units and the enemy to swap spawn locations. The most notable example is getting ambushed at Zofia Castle, which results in a unique map that you wouldn’t normally see.
Most people don’t seem to like them and I generally agree (although I’m ~90% sure that they upped the spawn rate for reinforcements in Echoes, it’s definitely not that bad in Gaiden). It could work as long as it doesn’t interrupt the story, but overall it just seems like forced grinding.