For one Aamir won't be part of it. And Ghajini 2 will not have anything to do with Ghajini 1.
This is because not Geetha Arts but a sub-division of the big production house, GA2 Pictures have registered the title Ghajini 2. The registration has happened for both Telugu & Hindi, not Tamil.
This is just like how Arbaaz & Sohail have registered Dabangg 3, Dabangg 4, Dabangg 5 & Dabangg 6 titles. The last time I checked, I remember till part 6 on the list. It could be more.
Anyways, even if Geetha Arts themselves had registered the Ghajini 2 title, even then the film wouldn't have had anything to do with first part. Don't think they've approached ARM either.