The coliseum should be easy enough to do. Perhaps, like the Dednaro (spelling?) mountains, the consecutive battles across the multiple screens.
The coliseum could be set up that way, battle, move the characters over a screen, battle, move, battle, move, BOSS BATTLE, celebrate, gain items, money, fame, fortune, etc...
Use the event editing creatively.
After each battle, write a unique value to memory somewhere. After the battles are won, or lost, have the event check for those bytes, and give items or money accordingly. The battles against the preset monster groups would be of increasing difficulty, and a boss at the end, down a long coliseum hallway, or connecting rooms.
Have a couple of identical locations, but different enemy sets.
So, you would have, say, 7 maps that look exactly the same.
The event would look at Crono's level in memory, and load the proper location, according to his level. Small level = the location with the easy monsters. High level = the location with 15 lavos spawns, 9 Magus', 2 Queen Zeals, and a Spekkio. >_> Or something.
That all came from the top of me head, and I think that was pretty creative. You could probably improve on the ideas, and tweak it to perfection, or change it all completely to suit your needs for some type of coliseum (which really is a great idea!).
I don't think I'll have time to put one in my hack, I plan on spending the most time on making a fully playable new character, with a new portrait, and palette, etc....
I may find time to toss in some prerelease stuff, though.
There will be a Poll over at Tek, for an unnoficial 'Popular Vote' and a judges panel for the 'Official Vote' for the winner. So there will be 2 winners at most, and everyone will have their hacks displayed in one thread, with screenshots, so everyone is really a winner, and have their work shown to everyone.