Yeah a show with the OT popping in and out is not gonna happen, even as teachers cameoing here and there shown to be mentoring new students. I think they will want to explore the beginnings of the mutant phenomenon and the early pandemonium and hysteria that sets in after 1973, leaving much of the now sequestered mutant populace coming to terms with their place in this new society. In all likelihood, it will be set after 1983 otherwise it may be a little anticlimactic leading into a movie like Apocalypse, which will be released prior to any live action show forthcoming. It's not going to be some far future, brand new team that has to face new threats, because the budget would simply be too high to do that justice on television, and you really don't capture the crux of the X-Men thematically, doing some far flung spinoff like that.
To me, this should be a late 80's/early 90's show. I am weary about including the original X-Men as weekly TV characters, but I don't think Xavier's school and his primary students have to be the focal point. You could focus on mutants around the world, and personal stories. I still think Xavier and the school needs to be involved, moreso than simply a backdrop akin to Arkhum Asylum in the ongoing Gotham adaptation.