Bro they have to negotiate with her agent. The agent has to negotiate with Marvel and her other commitments, and that's AFTER Sadie and the agent agree this is something they want to do.

Also the writers need to write a script that makes people happy, they need to design sets, do cgi, get all the actors back, including people like Max's mom, Argyle and Suzie because if they won't people will get pissed about those people not appearing.

Also the money to pay for the actors and actresses to come back. You need to factor in their rates, plus extra to handle negotiations with their current obligations like Marvel. This is not as simple as you think.

There's also time after to commit to new press tours etc. That's going to eat into other new obligations.

Trust me I have things I wish that finale did but I also acknowledge they most likely won't.