Also, I think last episode all but confirmed that Agnes is Agatha Harkness. From the cat references, to the commercial from early in the season, to the previews of her in a witch costume. All of this suggest it.
After watching more videos and doing some reading, I also think that anything Spiderman related is super unlikely to show up, since Sony still owns those rights.
That leaves Fantastic 4 and X-men as the most likely crossovers/introductions.
Considering this is based largely around the comic series "House of M", the X-Men have the obvious tie in. Also, Days of Future Past seems to take some inspiration from this comic as well, but I am not sure about that. Either way, the timelines match up. I just think there are just too many connections for the special cameo not to be one of Professor X, Magneto, or Wolverine. In the comic, Professor is the one holding back Scarlett Witch's powers, so he makes a lot of sense here. However, Wolverine is largely the main character in the series. Magneto's tie ins are obvious.
Honestly, it wouldn't surprise me to see all 3, given how important they each are to the story line arc in the comics.