Base is not very relevant.
It's the most overrated and overestimated aspect of MMA development.
What's relevant is what a fighter's focus is and aptitude for something in between the ages of about 16-24, on average.
No one would call Jose Aldo Jr a BJJ fighter although he didn't start training striking until he started fighting.
Junior Dos Santos was basically a wrestler until he started honing in his boxing skills with Luis Dorea.
good point, and hence also why I mentioned GSP. He's basically a wrestler with boxing these days. And Nick Diaz who is basically a boxer who comes from a BJJ background.