I'd like to see FE4 and 5 combined into one gigantic remake somehow, since they're basically connected, but I have no idea how they could fit it all into one game without it end up feeling watered down. Maybe have split paths along the vein of Sacred Stones, and let you choose between Seliph or Leaf's path?
That wouldn't necessarily work, since the end of Leif's path ends up crossing back up with Seliph's path. FE5 being a midquel kind of ruins things. It also doesn't help that FE4 and FE5 have
wildly different gameplay mechanics.
Yeah, they would probably have to rewrite the story quite a bit for that part. They'd have to stretch out the paths to have them converge much later, otherwise it would probably just be like an A path/B path kinda thing (which might work out alright, but, eh, whats the point, save for a little extra replayability)
Gameplay mechanics would be the big issue. Truthfully, at the time I wrote this, I was picturing the remake as using FE13's mechanics, but after rereading OP's post, I noticed he mentioned that gameplay mechanics would be kept intact. Honestly, updating the game mechanics is the only real point I see in making remake to begin with, so I guess I'll just say none of them should be remade.
Perhaps I should clarify: I would really
like to see this game made, if things like money and cartridge space weren't issues.
Realistically, I know it probably wouldn't work and they shouldn't try.
My hope is that after remaking FE4, which is bound to happen, that FE5 gets remade. And without gimping the difficulty.
I couldn't dig it up again, but I remember reading an old interview where one of the executives from either Nintendo or IS said he really wanted to do a FE4 remake, so I'd say the possibility is very real. The interview was a few years old though, so it's possible that idea just panned out as Awakening.