I have a thought for a Tom cameo playing the Dr. himself, while looking correct at his current age. We know that Tennant created a second Dr. with his character but that Dr. - while maintaining the same memories, knowledge and personality, was physically human (no regeneration) and this was the version that stayed with Rose.
Tom could play an older/future version of this Tennant era human Doctor. The current Dr. finds his way across the rift at a future point (a million explanations to make this work) and Tom, playing the "human version" tells him of his wonderful life with Rose, how they married, had children and then grandchildren. Then, one of their grandchildren comes into the scene and ends up as a companion.
The Dr. Back with his grandchild like the original Dr. Who series began! Only, it's Roses (and in a way his) grandchild.
Some wise-words can be shared, a touching moment, a funny bit where they fight over who gets to press some button in the Tardis or maybe a complaint that he was left without a sonic screwdriver and you've got a show that's come full circle with a perfect spot for Tom Baker to actually play The Dr (Proper) again.
I would keep this all to one show or a short two part story arch leaving the current Dr. free to go on with a current proper companion. However, within a short story arch, this would all be an Amazing bit in my opinion.
Anyway, that's the only thought I had on Tom playing the Dr. again while being age correct and actually being a meaningful part of, and furthering, the current story.
Love it? Hate it? Flame suit on. Plenty of tweaking needed but it could be done in a million ways. Just the basic premise of a human clone of the Dr. that's aged creates a spot for Tom to reprise the role of the Dr. for a moment.