I think that if Tatum and FOX are still fully committed to this film, and there has been no news to the contrary, I think the cameo in Deadpool being Gambit is a super high possibility. It makes the most sense, it builds for the future, and would be the most exciting option. The other most likely option would be Hugh Jackman. It would mostly just be for a big laugh but wouldn't really be setting anything up or a be a big reveal or anything. Gambit seems like the best idea.
I still think they are intent on making this movie. Tatum as almost put his film career on hold for a couple of years now. He could have been in so many large films at this point. I think they are working to get this done. Just my opinion.