For FFV, my favorite 2D FF, I'm not really sure on the direction it should take, since SE are supposed to be the devs who get paid a lot of money to figure out, but here would be my suggestions.

The only thing I think is a must-have is the overall polish. Tactics Ogre is the best remake I ever played, and it was funny how the team stumbled at describing it as a remake, reimagining, or upgrade, as they didn't really know how to categorize it. And that's exactly the kind of product that came out, something different than what we're used to. The big key for polish is making sure it does everything right on the system. It's hard to explain, but TO:LUCT took an amazing advantage of the PSP and widescreen, everything just looked so crisp and all the colors seemed right. Almost more than anything, the way the cursor moved on the menus and the sounds that responded to you all just felt so right. Of course it also means a re-balancing of the battle system, but I think the key is to make it more like a patch and less like a rework. The music also sounded similar, but so much work went into bringing it up to date with PSP standards, but then go above and beyond by not having any weird filters handhelds usually seem to have, but just sounding unbelievable through head phones. But the biggest part may be the artwork - keep everything how it is, but just make it awesome with the detail and things that the original didn't have, like shadows.

The only problem is if 2D overhead, even with all that would feel dated. Maybe there could be a slight tilting of the field or even a rotatable overhead camera like the DQ remakes, but the GBA remakes and FFIV PSP played fine with it, so maybe it could be left as is.

I don't think redesigned dungeons and castles is a good idea, though. IMO it defeats the whole purpose of the remake. I know you could have the same setting just with a new layout, but I'm not really interested in something like that. FFV had good level design when it was made, a positive quality like that is why the game deserves to be remade in the first place, it'd be odd to change one of its positive qualities.

I also don't think the 3DS would be a good idea, either. I don't necessarily think 3D is a gimmick, rather, I think it's so new that new experiences should be built up for it instead of remakes. If I get a 3DTV, I'm going to want to play Killzone 3, not God of War PSP remakes. While the 3DS can be a powerful platform without the 3D utilized, it makes it kind of a waste and that's a big reason why it's struggling for its voice right now in the market.

That's why I think the Vita would be the best choice for it. In addition to having a nice wide screen display to take advantage of, the OLED is really a "next-generation" screen. If you look at the 2D games that come out on Xbox LIVE and PSN, given what I've said above, I'd really like FFV to come out like that, just looking beautiful in its simplicity, with tons of different colors and shades to bring it to life, and on an OLED display, that'd really be something else. The Vita also has the awesome community features that could show it off. For example, you could post your favorite ability combinations or equipment setups, and rate/comment on them, or even upload them to facebook/twitter. You could maybe even use the NEAR functionality to see who's playing the game in your town/city, where they're at in the game, and what their party configuration is. Using stuff like the GPS to randomly generate a castle dungeon if you're in the city, a forest if you're in the country, or some kinda coastal jawn if you're on the beach. Not really sure about the camera functionality, but there's so many awesome things you can do with the network features, on top of the already obvious visual advantages for a 2D game like this. Something similar (minus network stuff) could work on PSP (as it's already been done), but I think putting it on the successor is a logical progression.

Another alternative would be what they wanted to do with FFIII. Originally it was going to be a PS2 project, which would have gone all out instead of being a showcase for a new handheld. With how far PS3/360 have comes, I wouldn't want them to spend the budget and resources to accomplish that (FFV isn't really designed well for that anyway), but I think a PSN and/or Xbox Live version that goes semi-all out would be fantastic.