Brock Lesnar is a former UFC heavyweight champion. There is a good chance that if he decides to improve his stand-up that he just might become champion again. It is hard seeing it happen against current belt holder Junior Dos Santos considering that Lesnar has a problem with striking, but it is possible.
Lesnar has also been a world heavyweight champion somewhere else though—the WWE. If things don't work out in the cage, he could always make a return at the end of his UFC contract.
In the end, it might be better for him, because with the amount of elite strikers that are there and those that are coming from Strikeforce, the division is going to get crowded. That, combined with Lesnar's inability to take a shot, is going to leave him looking weak and dispel the aura of invincibility that fans seem to respond to.
If he does cut ties with the UFC, though, it might help him to keep that myth about him and can even help him in the ring should he make a return.