If you do mean a dedicated server:

From my experience, it's not worth hosting a dedicated server. It'll usually just sit there empty, and you'll be wasting money on something that'll barely be used. The necessary dedicated servers are already there.

If you don't mean a dedicated server:

I'd suggest a less played, but popular gamemode. There's already plenty of CTF and battle servers out there. We don't need more servers, unless it fills a niche that the others don't.

Suggesting levels isn't something you can do on the flick of a switch. It takes time to find levels, and usually the best thing to do, is to find out for yourself.

If I recall, JJ2 works on mac but only on 1.23. But don't bother hosting with a mac at all, because the support with plus is nonexistent, which is practically essential for hosting. Without plus, making levellists and admin control is a real pain (infact, there's no admin control/commands in vanilla JJ2). Also, a much more limited gamemode choice. I can't join plusless servers either.

However, I recall Black Ninja was making plus work with mac, but there hasn't been any recent word on that.

I don't think there's much point in answering further if a mac is the only thing you have...so I'll leave it at here.

EDIT: I recall you can use WINE to get JJ2+, but I've never used it myself (and I heard it's complicated), so I'd be of no help.