Look at the HW division, it's severely lacking talent. You have JDS, Cain, Werdum and after that, there is a huge gap in talent. Brock being 38 fits right into the 'old' HW division. 9 out of the top 15 HWs are between 35-40 years old: Werdum, Hunt, Barnett, Arlovski, Bigfoot, Roy, Gonzaga, Mir, and Mitrione. Struve is the only top 15 HW below 30. I'm not saying Brock is going to take the HW division by storm [far from it], but he'll be a nice addition to a very thin division. I just hope he's been training MMA since going back to the WWE. If not, he's going to be in for a rude awakening.
For me, this has nothing to do with storylines, gimmicks or the WWE. Lesnar is an exciting fighter [win or lose] who can bring some needed interest to a stale HW division. Assuming, he's in good/top [MMA] shape, he'd be an interesting match-up for anyone not named Cain, JDS or Werdum. He's not going to win a title and he probably won't become the number 1 contender [unless the UFC gives him favorable match-ups], but at worst, he'll put on some exciting fights.