Unfortunate how the Brock situation worked out for the Broncos as he was drafted by Elway, was even a friend of his son and was groomed for 4 years under Manning. But as well all know, money talks and the Texans were willing to pay big bucks for a QB that has only started in 7 NFL games and only had 1 season in college as a starter, he's probably started 20 games since high school. Stats were not great last year, maybe in the 23-25 ranking as far as QBs go but is well liked in locker room and has good leadership qualities, plus the Manning under-study factor. He was drafted before the Kubiak era and I don't think he's a great fit for Kubiak's offense, not really that mobile and at 6'7" moves awkwardly and is a big target for defenders to key on also has somewhat of a sidearm motion and gets lots of balls batted even at that height.
Broncos were high enough on rookie Trevor Siemian from Northwestern to keep him on the 53 man roster last year, he's got a better arm and mobility so likely a better fit for this system, but very green. Rumor has it that they are going after Kapernick from San Fran who would be a good fit for this offense will his mobility and arm strength. I guess RG3 would be another option, but his rocky relationship with Shanny who is still close to Kubiak might make that move not as likely.