Christian is a very classy individual and from all accounts left WWE on excellent terms. Of course he has a lot of friends there (Edge, Chris Jericho, Matt Hardy, Jeff Hardy etc.) and I remember reading that he left on really good terms with Vince McMahon as compared to other wrestlers who have left (Booker T, Kurt Angle, Goldberg, Lesnar).
I have no doubt that WWE would be sending him some feelers and would get in touch with him once his contract with TNA runs out. One the flip side Christian has also stated that he's really happy with TNA and being part of an up and coming organisation. But then again he hasn't been booked the best in recent times.
Interesting times ahead, he could still sign again with TNA but a WWE run around the corner could be golden. Potential feuds with CM Punk, Chris Jericho, Shawn Michaels, MVP and Edge could be absolutely out of this world promo wise and could produce some really great matches.
In terms of comparing Christian with the other greats you mentioned, I'd rank Christian as being a better pure wrestler than both Edge and Triple H. Sure Edge has a really great character now and his promo work and in ring work has been outstanding, but from a personal point of view I believe Christian to be the more complete in ring performer.