It's a shame Tennant is the one who hasn't aged, because I suspect, since it's Rose, it will be her Meta-crisis Doctor, not the Timelord who Regenerated into 11. That would allow for no explanation necessary for the aging.
Based on nothing more than the fact it would probably be the easiest to do I imagine it'll be DocTen and Rose during their S2 adventures.
There's so much we don't know that if they got Eccles back she could actually be with him, and Ten alone or with Martha or Donna.
Too early to tell how the logistics will work.