No one can really answer that question for you, as we all have varied and diverse interests. You need to ask yourself the question "Why did I want to root in the first place?" What sort of things do you want to do with your phone? Have any hobbies? Into sports? Are you health conscious? Do you want to do your banking on your smartphone? Are you crazy about music? Then go look in the Market along those lines.
As for themes, my favorite app is Fancy Widget, which is free. There is also LauncherPro, ADW, and Zeam, as well as a bunch of others I've never tried. All of these launchers have special features which make them superior to the stock launcher. Again - search the Market.
Best of luck to you. I'm sure if you expend a little time and effort browsing the Market, you'll end up with a phone that is uniquely you. And that's really what rooting is all about in the first place.