Here's the list so far. Some of these are very easy, and some are actually incredibly ambitious (and understandably passed on), but they were what came to mind for a general "FF6 Improvement" and are on the list just for completeness. Feel free to disagree with some / none / all of these :-)
* De-capitalize all NPC names. The player character names are de-capitalized, but not the other NPCs, so you get dialogue that looks like this:
Terra: Blah blah blah.
ARVIS: I concur
Terra: Indeed
It looks unpolished.
* Change GP to Gil for consistency with newer final fantasy games (and all the remakes)
* Where possible, change item / spell / character / etc names to their FF6 Advance equivalents (in keeping with modernization. Also, I trust that translation more than the SNES one. Speaking of which...)
* Replace the dialogue with the FF6 Advance dialogue. Note: I understand this is a mammoth task and actually recommend AGAINST doing it, it's just on this list for completion's sake.
* Rename the monster Umaro to something generic, like "Yeti", so that the game doesn't show his name to you before you get the chance to name him. Speaking of "Yeti" - the game tends to call him a "Sasquatch" all over the place, but "Yeti" is more accurate.
* Restore the control config screen. I don't know how this would be done, but some other hacks have done it. The functionality is still in the game, it's just the menu selection that's been removed.
* Reverse the B-Button sprint implementation (would be better as Sprint by default, hold B to walk).
* Expand character names to 8 letters. Very ambitious, but only because the relevant menus would have to be altered so that the character names don't spill into the borders. Outside of that, expanding to 8 letters isn't that tough.
* Expand item / spell / esper names wherever possible / wherever easy.
* Revise the locations where the Prism Cape is sold. It makes sense for the Sprint Shoes to be in a bunch of unrelated shops in the beginning of the game (like the Returner item shop), but not the Prism Cape (which shouldn't be treated any differently from other relics and should only appear in relic shops).
* The Prism Cape is potentially too inexpensive for its effect. Should probably be 2000+ in price.
* Several daggers are still valid for Runic and 2-hand (mythril knife is one of them).
* Make it so the Chainsaw won't attempt the insta-kill attack if the enemy is immune to Death (I think there's a patch out there for this already...?)
* Consider making the Thief Glove change *Attack* into Mug instead of changing Steal into Mug. This would be neat and more useful, although there are probably downsides (the offering and thief glove might not work together, for example).
* Make it so Shadow always survives the floating continent, even if the player doesn't wait for 3 seconds left on the timer.
* Stub in some commonly requested features for hackers so that making hacks based on your patch is easier (I did this in my Earthbound Tweak and War of the Lions Tweak). For example, implement natural magic for Strago, but keep his natural magic table blank for people developing hacks to fill it in. Or increased HP room for monsters (for more than 65000), but don't raise any monster HPs. I guess expanding names fits in this theme too.
* There are other misc bugs that are still in the rom that wouldn't be difficult to fix. For example, if you leave a rat alive in the attic of the opera house, then come back and battle that rat way later in the game, the game gets all kinds of screwed up. It'd be easy to insert an event that removes the rats at a certain point to fix this bug (I could do this for you). It might be worth puring over Zed's buglist to find other bugs (not already addressed in FF6I) that could be fixed relatively easy in data.
We have EVEN MORE themes now, but some old ones are gone...