I think there's scope to someday have two incarnations appear in a series of stories together, without it being a gimmick for an anniversary.
Imagine a normal series, just The Doctor and the companion having adventures, but in some episodes we get a glimpse of another Doctor, watching, or a cut to them in their TARDIS, talking to someone cryptically. They could even have the companion interact with them briefly, unknowing to his true identity. And this would build towards the finale, which would be a big, multi-stranded narrative, with the companion being our eyes, discovering what the big threat is they are trying to prevent.
I don't know, I just think that whilst Doctor Who is all about change, they don't play on their history enough at times, when there is potential to have some varied and generically unique stories. Even past companions are always open to being explored as a way to further explore The Doctor and how he influences those around him, and yet they never seem to hit that as powerfully as they could.