BK wrote:Control in what way?
If with some knowledge of the need to regenerate comes, a Time Lord or Time Lady can control it. Training helps here as well, as in the case of Cho-Ji whom projected a future version of himself prior to regenerating.
Simms could come back, first he could have done some time traveling in the Tardis prior to settling in at the time he did plus, he could just be possessing a body and thus make look like himself.
What I'd like to see is the Master deciding to undo the Timewar, to bring back the status quo prior to it. That could be fun to see, plus that the Time Lords aren't gone, but just hiding between the universes. IMO.