Considering that Brock Lesnar's illness is right now very serious, his immediate future is being confined to a hospital bed. However, the predictions I have are under the situation that he is perfectly healthy. So while none of this may happen it is interesting to consider how he would do against the top contenders of the UFC Heavyweight division. Obviously the first man in line to try and take his belt is Shane Carwin. Now Carwin does have amazing wrestling, whether or not they are on par with Lesnar's NCAA champion status remains to be seen. Carwin has devastating power in his hands and out of his 11 wins, 6 are knockouts, and 5 are submissions (one of which you could technically think of as a TKO because it was a submission due to strikes). Carwin also has largely the same physical stature that Lesnar does, being 265 pounds, having an 80 inch reach, an being 6"1' (Lesnar has an 81" reach, is 265 lbs, and is 6"3' tall). No doubt it would be a very fair and amazing fight seeing both wrestlers prove their skill. Their incredible power is arguably even, however it is important to take into consideration that Carwin has more experience with 11 fights, all of which are wins. Lesnar only has 5 fights under his belt with a record of 4-1 (although in my opinion his first fight against Mir should have been a win and Steve Mazzagatti is a moron!).
Now if Lesnar were to win his next contender would be Cain Velasquez, a devastating wrestler whose pace is unmatched in the Heavyweight division and whose relatively small stature is very misleading as an index of his power. He has no problem with lifting 265 lb men over his head even while he weighs 230 lbs. However, knowing that these men are both phenomenal wrestlers and that Cain's pace will ensure that there will be no moment for Brock to rest in any of the 5 rounds, Lesnar has never actually come out of any of his fights gassed. After every round Lesnar is extremely composed and calm. And while Cain has amazing strength, the fact that Brock is bigger, stronger, and has the same, if not better, skills that Cain has it is safe to argue that Lesnar could stop Cain in his tracks.
Now with Lesnar in a very unhealthy status, suffering from Mono and a serious case of Diverticulitis which requires surgery, Dana white may schedule a Interim Heavyweight Championship bout between Cain Velasquez and Shane Carwin. My opinion between that fight is that it will play out the same way as if Cain fought Brock, except Brock is always continuously pushing forward and he hardly gets rocked. I have no problems seeing these fights unfold in any way and they would all be exciting bouts.