Which uninvited guest(s) would you like to see crash the party? Go with whatever reason you want. Shock value, maximum drama, story potential, etc...
Adriene Frost
Madelyne Pryor
Cassandra Nova
Destiny
Moira X
Stryfe
Scarlet Witch
Lilandra
The Phoenix Force
Thor
Nate Grey (X-Man)
Deadpool
Mojo
Miss Sinister
Saturnyne
ETA:
Maybe the Phoenix Force will crash