So, after poring through multiple older school and newer school rulebooks - the ones I mentioned here, mostly - I ended up deciding to run this campaign using Tales of the Valiant. I think this is the best mix of the new and the familiar for my crew - just enough new to make it interesting without having to learn an entirely new system. Most of those other systems I checked - like 13th Age, Level Up, PF2E - are all candidates for something down the road but this is the plan for now. ToV is probably 75% 5th Edition so it's easy enough to fall back to those rules if I encounter a gap and feel the need but I don't really see that being a problem.
On the setting side I decided on ye olde Temple of Elemental Evil, set in Greyhawk, pretty much following the original setup. I'll be using the Goodman Games 5E adaptation as the main document but I will play around with using the ToV Monsters where they are relevant.
My players are excited and already looking at character options even though the start for the campaign is a month away. I've warned them that even though it's a 5E version the caretakers at GG did not go through and rebalance the encounters - if there were 12 Ogres in a room on the second dungeon level in the original then they are still there now - just with updated stats. We played through Goodman's B2 and X1 a few years ago so they know it will be different than a modern WOTC adventure. This should balance out to some degree as I have 8 players and even if they don't all show for every session odds are I will have at least 6 most of the time.
I am excited too - I haven't run Greyhawk in quite a while and I last ran the Temple in 4th edition - hard to believe that was 13 years ago - and I am looking forward to re-introducing the world and the adventure both. Greyhawk was the setting for a lot of my 3rd edition games - including a lot of time spent with Return to the Temple of Elemental Evil - and at least 3 of my current players were around for those days.
So there's the new thing for us. I'll be posting more about here as I prepare and as we get going.
