So I've been playing since Beta 3 or so, and have noticed the same issues a lot of other folks have noted w/r to heroes making trained units irrelevant. So my thought was, rather than fighting that tendency how about tweaking things such that your heroes grow into Generals and bring units along in power with them rather than obsoleting them.
1) Add more skills like ShieldWall, Obsidian Guard, Tactics and the like. I already choose Defender on all my warrior types explicitly to get access to the stack buffing skills that have already been implemented - just to try and make trained units relevant. But imagine an Assassin ability that adds Master Scouts to all units in the hero's stack. A Warrior ability that can add "Bloodthirsty" (whichever unit ability adds 25% damage to already damaged units) to all units in the hero's stack. Or a stack equivalent to "lethal" that adds attack to every unit in the stack. A Governor ability that reduces the upkeep of all units in the stack. Mage abilities that add resistance to fire/ice/lightning to the whole stack. Etc. More abilities like that - particularly abilities which enhance the offense of trained units - might encourage heroes to grow into leaders of troops rather than simply one-man wrecking crews. Particularly if the abilities are "per-figure", so that larger units take better advantage of them than other heroes. My preference would be for none of the "enhance my stack" abilities - even the current ones like shieldwall - to apply to other heroes in the stack.
2) Allow units to gain abilities/traits as they level. It doesn't need to be every level like heroes, but even every third or every fifth level would make a L10 Spearmen unit far more differentiated from an L1 Spearmen unit.
3) Allow me to cast unit enchantments on trained units. It was almost SOP in Master of Magic to enhance your best units with nifty spells, and FE has plenty of spells which could help on spearmen and the like as well, but I don't appear to be allowed to stack Stoneskin and Flame Blade and the like on trained units. Why?
4) Add more high level spells that function as Mass buffs. Imagine a "Mass Stoneskin" that applied stoneskin to every unit in the hero's army. Or a "Mass flameblade". Etc. It could be strategic like the current spell that increases movement of the whole stack, or even simply tactical. But something to let high level Mages leverage large groups of allies more effectively.
5) My one suggested "nerf" such as it is, is that rewards for taking out Monster lairs often seem way, way, way out-of-whack with respect to the difficulty in defeating them. I have found a lightning bow after defeating an Ogre early in a game - which pretty much single handedly turned that hero into a godly killing machine that slaughtered everything in his path until I grew tired of the overkill (+2 per level lightning damage? Really?). Or heavy armor from killing a Harridan (whatever the huge spiders are called) which made me practically invulnerable to any trained units in that stage of the game. Just one find like that becomes game-breaking in a hurry. If I had to take out three Fell Dragons or something to get that kind of power at least I would feel more like I earned it. And I feel like quest rewards and such should be better than what I can simply build, but not orders of magnitude better. Maybe a limit such that Quest rewards can be no more than a few steps down the tech tree from the current leading player(s).
--Zeno