How about the use of a device designed to DEGENERATE the Doctor?
If he is functionally immortal, then as he was previously speed-aged and then made younger again...The Doctor degenerates back into Matt Smith and viewers expect it to end there and Matt's Doctor to go about his business, but he continues to degenerate. Faces of available actors seen, the Doctor slumps to the ground and from behind we see him degenerate all the way back to Hartnell look, and it doesn't stop there...still.getting younger, his first body shrinks as he becomes a boy, a toddler, a baby...baby shrinks from 1 year old towards newborn size...meantime associates and companion have been trying to get into a control room, defeat guard and reverse the process...machine breaks down when Doctor is in Matt Smith stage again...machine is fixable but Matt Doc says "wait, let me have a little fun first"... fun times fir some time...sudden mass dalek attack...time war scenario...Capaldi hasn't gone back to help yet...Smith Doctor tries to puzzle out what to do...blasted by dalek, not happy at such a short time as a former more youthful self he dies angry...regenerates back into 'furious eyes' Capaldi..."Normally I'd need a bit of a rest at times like this, but oh...I am so absolutely LIVID RIGHT NOW!...heads into Tardis to join his previous incarnations saving the day at end of 50th anniversary special, filmed from Capaldi's pov principally. Sorted. 'Long cameo'
for Smith, the on we go, as before