If you want a lot of conversion friendly guys, Chaos could be for you. They'd probably be my first recommendation as they're both very aggressive. CSM play different than loyalist SM, but I don't know how much different you want, they are still guys in power armor with a few extra tricks for the most part and tend to favor CC over shooting. (flame shield on)
If you like having lots of color schemes, consider Eldar, they can also be made out to very aggressive if you value speed over durability.
'Nids just got a new codex and can run either way. Their guys have less individual "moddability" than Marine variants will have, but I think they have viable and fun CC/rush and shooty builds available to them. They're also significantly different to play than Marines.