Much as I'm looking forward to seeing Capaldi... I have to admit it would be fun to see more adventures with Tennant's Doctor.
It's a shame Moffant and/or the BBC don't see the value in bringing back the previous Doctors for one-off specials. A new hour-long story with Seven, Eight, or Ten would surely pull in huge ratings, and would still leave plenty of time in the year for the current
Doctor to make his mark.
I really doubt audiences would be confused by the idea or anything.