Personally before he calls it a day I'd like to see him return to Aprillia and take the premier class title back to the company that gave him his first championships.
Unfortunately Aprilla no longer run a MotoGp team, and are unlikely to in the near future due to financial contraints.
If he was inclined to change manufactuers again, then Ducati would be the obvious choice. If he won the World Championship for them, he would be raised to heights above Godliness. However in the past Ducati has tried to get his services, but failed, and for reasons I don't understand they have just signed up Gibernau.
Rossi to Ferrari?
Well the money wouldn't be a problem, sponsers would be queueing from Modena to Rome to have their name attached to Rossi/Ferrari.
Motivation? I can't see Rossi retiring at 27 and just partying for the rest of his life.
Ralleying? Had a go on the RAC a couple of years ago, and crashed out on the first stage, since then he has been a little quieter on the subject.
Skill? Time will tell.
Do Ferrari want him? If he has the skill to be at least competitive, then they would sell their Grannies to get him.
Some of the motorcyling press seem to be convinced he can match John Surtees' achievments, if he can, in the motor sport world of the 21st century, then he would be worthy of being crowned the greatest rider /driver ever.
And that is the sort of challenge that Valentino Rossi will not be able to resist, if he thinks he can do it.