When do they lists special guest?

I'm trying to overwhelm a friend with good reasons to go and I'm finding it difficult to find specific info.

He's big on Hero Clicks, comics, and Champions rpg. If you know of anything in those areas I could use as ammunition please let me know.

I always like to show people interested in the convention the events list for things they are interested in:
HeroClix
Champions RPG
Champions RPG/Hero System

Admittedly there weren't a ton of events for Champions in 2019 but HeroClix looks fairly active!

I am also a big fan of this blog post which stays up to date. It doesn't go into exhaustive detail about any specific topic but I think it gives a good overview of just how big the convention is and how much is going on.

Keep an eye on the "Experience" and "Press->Press Releases" links on the main page.  They will usually announce special events & guests there first.

I admit I have had so little time in the Exhibition Hall the last couple of years I am not sure, but I definitely saw some of the designers of Champions in the Hall here and there over the years, so that basically counts as a celebrity sighting as far as I am concerned :)