know it's not on the list but SS13 would be cool
-all dev works pretty much in place I'm pretty sure you can already use the highly developed/established ones which are constantly being worked on, are open source and have a dedicated wiki and dev community (like tgstation), has a large pool of maps, etc.
-requires little staff intervention since most rounds play themselves out, by being roundbased it also means shit like cheating/morons have a relatively low impact
-games so old that it probably wont use a lot of resources anyway so the cost of running it won't be very impactful