The first Forza game with bikes is the first one I probably won't buy.
I'm going to say the same thing I said of the notion of adding motorcycles to Gran Turismo. If they do ever add bikes, I want the player properly punished for using them. What I mean by that is I don't want your bike and my car to make contact and you cruise along relatively unscathed as though it was car-car contact. Realistically, my car would decimate your bike without even trying.
If your bike makes contact with another car, with a wall, or even with a curb in the wrong way, I want you face-planting. I want your bike skidding several hundred feet down the track and into the grass somewhere while your bruised body skates off in another direction for a hundred yards. Once you've come to a stop and I'm half a mile ahead of you down the track, you take a few moments to collect yourself, scramble awkwardly to your feet, and then begin the jog back to your bike. Then you have to stand the bike back up, mount it, and start it before you can even think of rejoining the race... as I come around to lap you.