I'm glad this is confirmed and that it can finish out the trilogy. As for who his villain can be, how about adapting Whirlwind to fit Hope instead of Janet's stalker.
I like Whirlwind but I feel he may be a bit too small scale as a solo villain for a whole movie.
Whirlwind as a henchman or part of a group like Masters Of Evil could work.
My idea for Ant-Man 3 would be AIM vs the Ant-Man gang.
Monica Rappaccini was running AIM's European operations (creating MODOK) and after the Aldrich Killian's death they evolved into techno-anarchist's under Rappacinni and Modok's leadership.
Mitchell Carson from the first Ant-Man film sell the Cross particles he took to AIM who are trying to use them for their own ends but reach a stumbling block without the Pyms expertise. The Pyms are back after End Game and AIM kidnap Hank & Janet to force them to work on Pym particle technology for AIM. Scott, Hope, Bill, Cassie, Kurt, Dave and Luis come up with a plan to break into the AIM Island base that is holding Hank and Janet to rescue them and stop AIM.
The Ant-Man and Wasp team break into the AIM base and end up having a big battle. You can have AIM in their beekeeper suits with all their high tech whens like laser guns, rocket packs, mechs, ect versus the Pym/Lang's with their size changing, insect army, blasters, ect.
They could include some supervillain henchmen like Whirlwind, Atlas, Black Ant, Beetle,
Doctor Nemesis, ect who got their powers from experimenting with Pym Particles.