It's this possible?

Create a item to revive Leo and add him as a 15 playable character.

We almost got there, see https://www.ff6hacking.com/forums/forum-52.html.
PowerPanda finished the thing and has a 15th playable character in his hack Divergent Paths. I don't think he documented the whole 15th character thing but I could be wrong.

Add a secret room at the Colosseum with a secret boss.

This will require map editing and problem is that you might exceed available tilemap space. There is FF6LE-CE that can expand tilemap space and possible FF6Tools too with modified json setting file and expansion done manually.

Put the Dragons that were on Kefka's Tower in other locations.

If it's only a matter of relocating the NPCs it can easily be done with FF6Tools or the regular FF6LE.

Add new bosses in the places where the Dragons used to be.

Pretty much the same reply as the map editing one but instead you might run out of NPCs spots. There's a few you can add without expansion though.

Make the man in Narshe only give you the Ragnarok magicite and give you a hint about where it's the sword.

This is event editing and can be done easily.


If you plan on expanding things like tilemap or NPCs an expanded ROM is the easiest way to go.