Well, to be fair there seems to be a pretty big gap between popular and not popular. If we're looking for another server that'll peak users, we'd probably need another 24/7 server covering a map we're already covering that's popular. It's pretty obvious what attracts the masses, and things that are "different" usually don't. If they did... then everyone would be doing it already.

If we want to try something new that'll be popular... well. We need to take chances. Throw up a promising new mod and see if it takes with the overall TF2 community.

Speaking for myself, there's probably not much that'd pull me away from Tempest 2. The first thing I thought of was different gravity settings simply because 2Fort w/ low gravity might attract me away from Tempest 2 for something "different".

I absolutely think the servers need to have predictability in IP addresses and maps so players can know immediately what they're connecting into.  If we have six solid ideas for servers, I would say we probably don't need to "waste" one on an experimental server. 

That said, adding untested custom maps or mods (especially ones that have the potential to drastically modify gameplay like tripmines or medic infection) to existing servers can be risky, so a place to test out players' response to them before incorporating them more formally might be useful.  The Tempest servers, compared to a lot of other servers out there, is pretty conservative when it comes to alterations to gameplay (such as instant spawn, low gravity, or off-the-wall mods), so we should be especially careful before introducing new elements.  Finally, people might occasionally want a break from that normalcy without sacrificing the other stable servers.  2fort with party hats and low gravity might be fun once in a while, but I wouldn't want to see it become a permanent part of Tempest #2.