I like the Minotauro combo: boxing and bjj.
The most important things about boxing are head movement, footwork and striking defense. If there are any three things that are lacking from much of the striking I see in mma, then it is those three things.
There is very little head movement in mma (or K-1 for that matter). And I'm not talking about Joe Frazier style bobbing and weaving that works for boxing but is instant-knee sandwich in mma. I'm just talking about simple side to side, chin tucked head movement--maybe with a little level changing. A little of this would go a long way for most mma fighters I've seen.
Footwork speaks for itself. Striking defense--if you are primarily a ground fighter--means a couple of things: (1) keeping your hands up, (2) knowing how to avoid punches--especially when on the ground on the bottom--and (3) being able to strike JUST ENOUGH to close the distance, work a clinch and get a takedown. Too many ground guys land a punch and think they've just turned into Jake LaMotta or something. Rickson is excellent on this point--as is Eastwood: a man's gotta know his limitations.
BTW, where I train jiu jitsu we work takedowns every session. We drill about five throws for ten-fifteen minutes (the same ones every class), then go attacker/defender for a little while, then go "100%" takedown-only sparring for a little while. THEN we start working on the ground. So, for me, jiu jitsu takes care of my ground and takedown fighting.