I have noticed that the Coliseum is a nice place to PvP once in a while, but I'm certain there are some ways that it can be made better. Here's a few I might suggest:
1) Rooms. I've noticed that there are only a few settings such as "Tier 1," "Tier 2," "Tier 3" and "Ultimate" Brawl modes, but the only other information you can see are the entry fees and the number of people waiting. You can't see who is waiting, and you're forced to wait until the game starts, you can't start it yourself. Making rooms for PvP, or at least rooms within the limitations of the current Tier system, might make PvP a lot more enjoyable and a lot less tedious.
2) Free-Play/Practice. There's a chronic lack of a method to practice your PvP skills, and I'm noticing that EVERY mode costs crowns. Not everyone who plays is a gambler, they might not want to risk their funds. I think there should be a Practice tier where people can fight without any reservation. The catch would be that, should there ever be leaderboards, that Practice will NOT count towards it.
3) Teams. Only having free-for-all modes can get somewhat bland after a while, and some people -- i.e. guilds -- might want to team up with each other.
These are just ideas. Feel free to critique them, or perhaps even add your own.
Ah yes, a mode with no entry would be nice, for those time you want to do some pvp, but not competitively,just for fun.
Or if you want to test something.
Probably also with no time limit or points or anything.