I feel like starting a roleplay character who I want to be Enclave, but is there really a way to do that in the game? I know you can't exactly follow the main questline that way, and the Enclave are hostile to you anyway... Any advice?

Play as an Enclave Outcast. You could find commiseration with the Brotherhood Outcast this way. At least someone will not hate you.

If your on PC, there are some mods that let you play as Enclave, like Enclave Commander, among others.

Playing on the 360, I should be getting a hard drive today and thereby have access to all of the DLC...

That being said: Possibly RPing as a brotherhood of steel member or someone who wants to be? Startign with operation Anchorage, then able to use Broken Steel later? Thoughts? Help?