The main issue there is the gap years, contrary to popular belief, are not done because the actor playing the Doctor wants to do other things, but because of the showrunner. For example, the 2009 gap year did not happen because of Tennant's Shakespeare thing, but rather Tennant pursued the Shakespeare gig because he knew Doctor Who would be taking a break in 2009. RTD made the decision to take 2009 off in 2006 and Tennant was informed around that time as well. Which is why bringing Paul McGann back for specials or some other interim Doctor wasn't a practical option then.
Likewise, Capaldi's gap year came as a by-product of Moffat preparing to leave in 2015, but by the time BBC convinced him to stay on for one more season, there wasn't enough time to get anything on the air in 2016. So likewise, "Moffat written McGann era specials" could not have happened under those circumstances either.