These are just a couple things that could be implemented easily that would make the game that much better.
1) I know I'm beating a dead horse with this one, but Health Bars, at least on Bosses. Everyone wants them.
2) Equipment sets. Because you have an infinite inventory size, there is no need for alts, so you can be every role you want to be on one character. Because of that, you end up with tons of gear, and changing that gear can be annoying, especially if you realize you put your 1 star gear on instead of costuming it on accident. So what you could do is enable custom equipment sets. You could arrange your equipment once, save it as a preset, and then next time you want to switch to that equipment, you go to your equipment sets and select the set to equip, not the individual pieces.
3) Chat tabs. It's a super minor thing, but being able to split off zone/guild/party/etc... chat into their own tabs would be awesome.
Don't assume EVERYONE wants health bars on bosses, I don't, and i think it would detract from the game.
Chat tabs would be good.