You could say nostalgia but the movie seems to be taking some cues from the 2009 game which a lot of people consider the true third movie (before this was announced at least), that also had Gozer in it. The trailer throws out some Gozer elements with the shandor mine and the terror dogs so it seems fitting that stay puft should also be in there somewhere as that was Gozers last avatar on earth (supposedly can only appear as that avatar once its been chosen). I would imagine somewhere along the line the mini pufts combine to make the larger Stay Puft.
It's up in the air if Gozer is the bad guy or it's something else that's a bigger threat with Gozer being relegated to minion status.
