|This post doesn't have anything to do with Storm. I just like the picture.|
Colossus is another character that I like playing with a theme build. Most people play him as a tough melee character, doing damage largely with his fists. He has a set of powers that focus on his friends, though. He can do the "Fastball Special" Wolverine throw that is a staple of X-Men comics, and Wolverine will stick around to fight for a while. Colossus can also summon his sister Illyana (aka Magik) to fight alongside him. With the right gear and build, Colossus can have both helpers out and fighting all the time, turning him into more of a three-person team than a single character. It's not the most optimal damage-dealing setup, but I think he's more fun to play that way.
The most customizable Marvel Heroes character, by far, is Rogue. She has a set of powers of her own, but nearly two thirds of her power choices are borrowed/stolen from other characters. There are dozens of possible choices; honestly, it can be a little overwhelming to decide what to focus on. Rather than focus on pure damage output, I created my Rogue builds around themes. My first build was focused on fire powers and gear. After a while I decided I wanted to try another option, so I created a build using all X-Men powers and gear. (A few X-Men enemies in there, too.)
I find that playing Marvel Heroes can get repetitive if all I do is focus on "optimal" character builds. (True of just about any ARPG.) Playing around with theme builds as an alternative helps me to see the game from a fresh perspective.