Here's one of mine.
Enemies with skills. Some 40% of enemies too. Any class of enemies can have them. Could be anything from Miracle to Nihil to Adept. But to reduce the big hassle of checking each enemy's stat screen, you press a single button (could be Select) and you see the enemy's skills icon(s) blinking on them, kinda like the shield with bosses. Having enemies with Parity would make it more challenging indeed. Some units get way too dodgy from supports and such.
Would also be nice to see how many turns you need to get to supports.

3rd tier, though RD has that. Balanced "S" weapons(Vidofnir being less f***ed up) for TSS.

Perhaps make killer or brave magics. That would be nice.The idea of seeing how many turns are required to have a support would be great, though the universal rule is that it take a maximum of 42 turns to get a support. I a way to recruit enemy units would be intersting, like the Mediator class on FFT. It could be an innaccurate method (in FFT the skill "invite" which recruited enemies had a 30% chance of working and much often lower) that obviously excludes bosses.

That.... would... be... so... fricking... EP1C!

More FEs like the GBA ones.

But, far more similarly,

Magic Lances/Axes. Sword = overrated

Is there something that the GBA games had that the consoles didn't by chance? >_>

like sucking

Larry, then to make up for that it would only make sense for Swords to get physical 1-2 range weapons.

What about different mugs for first and second tiers, a la FE9 Ike? Obviously wouldn't work for FE8, but people with fixed promotion paths would work nicely. Probably could only be implemented on lords or people with set-in-stone forced promotions that you couldn't cop out of, though. And would take up a helluva lot of space.

I hated Knives, though, but maybe swords could get a ranged division like a throwing knife. Or even a boomerang... even a shuriken with 2 or 2-3 range (because it'd be awfully hard to hold it and fight melee with one of those things).

Enemy faction colors, too. I hated how every enemy soldier was always red. Does Bern honestly equip their knights with the same orangey red attire and armor as does Laus and Caelin and the Black Fang and everyonefreakingelse you fight in FE7?

I like Asvel's idea of having knives as throwing weapons. Seriously, making them thier own weapon type is awful. Knives make me think of thieves, which makes me think it would be cool to have a full "steal" animation.

With magic, there oughtta be tomes that hit from 1-3 or 2-3 spaces. I would think that a mage can throw a fore ball more than just 10 feet >> Or at the very least, have more magic period. If there can be 8+ weapons per weapon type, why not with magic?