Lego Dimensions has already received tons of expansions, adding much loved iconic characters to the already jam-packed game. TT Games doesn't seem to be ready to stop though, and they recently revealed through a very short teaser trailer, that more news regarding future expansions will be revealed on June 9th.
There's already a rumour circulating though, and Press Start claims to have a list of these expansions. Take this with a grain of salt though, as they don't cite a source:
Sonic
Harry Potter
The Fantastic Beasts & Where To Find Them
Adventure Time
Ghostbusters Reboot
Mission Impossible
Teen Titans
The Goonies
Powerpuff Girls