I'm sorry your core GM passed, but finishing the campaign is a great way to honor the years together. That's awesome.
I've considered going back to our own Hellmouth, but those players are all scattered to the wind. Still, it'd be nice.
That's why I'm leaning toward running Exalted. One of the other players at my table is my brother, he is the only member of our old group still playing with me. We ran a nearly four year Exalted campaign that spanned both groups, with both he and I running for big stretches. It's been 20 years since that campaign began, and we're fast approaching 20 with the current group. It feels appropriate to go back to where it all started for our current players.