wow gawd there's so many!! I feel like they won't be able to show a lot of these reunions unless they do it flashback style in like a mini- or anthology series. So many of the characters have been active for months, it'd be strange to think they haven't reunited by now. Big ones for me:
Synch and all the GenX-ers.
Emma and the original Hellions is a huge part of her story so this was a missed opportunity.
Have Betsy and Warren interacted? Neither of them were resurrected, but they were kind of a couple in the last Uncanny run, no?
Betsy and Doug.
Cassandra Nova??
Banshee with anyone! Banshee and Forge! Best buds who were always relegated to "reserve" X-Men.