I think Lesnar is improving but not at the incredible rate that some people seem to think he is.
Did he improve in his second fight against Mir? Yes, but when you face the same guy twice of course you're going to train more for his style and get better sub defense when you just got submitted. It's not like Brock was doing anything different the second time around with Mir other than some better hip positioning and wrist control. Other than that I've seen nothing to make me think he'll be champ for a long time.
When I think of improvement I think of a guy like Rashad Evans who started off as a typical wrestler and has now developed into a more well-rounded fighter. But, when I see Brock I don't see any of that. The only thing I see Brock improving on is his top game and sub defense. I can't imagine Brock ever wanting to stand and trade or going for submissions.
But, this could all be just speculation as we don't usually see Brock do much else other than wrestle and the guy hasn't had many fights to go off of.
Don't be shocked to see a sub finish if this fight with Carwin ever happens. He could be a beast with arm shoulder locks. I could also see a guillotine if he catches Carwin shooting in.
Take a look at his fight so far.
Min Soo Kim - over in like 45 seconds, no chance for a sub.
Mir I & II - why sacrifice position trying to go for a sub against a BJJ black belt
Herring is notoriously hard to sub.
Same for Couture, plus as is wont to happen when 2 great wrestlers face each other, most of the time on the mat was spent in scrambles.
Like you said not a lot of fights to go on with Brock. But look for him to expand his arsenal as he starts facing the other wrestlers (Carwin, Cain) and strikers (Kongo, Berry).