Interesting games. I remember trying Stendhal way back, like 20 years or more. It's neat to see that that is still around. I vaguely remember killing rats, lots of rats! I have of course forgotten how most of it works and I'm sure things have changed since then!
I tried a bit of Isleward too and just finished the tutorial after which I made it to the main map.
I think we'll need at least one person with some experience to play either of those as a group (hint hint! 😉) since they are games that have a lot of depth that you don't generally pick up on the fly. We had a similar experience with an earlier onFOSS event where we played Veloren which worked well because there were two or three players that knew what they were doing. I would have been completely lost without that!
Wesnoth could be fun. I just tried this out quickly and it looks like the turn-based system is such that you have to wait for each other's turns. So only one player can do actions at any given time. I fear this would make it too slow if there are more than four people or so.