I know this isn't too much/not at all related to custom campaigns, but I was going to link this page to the CCI. I feel that people who enjoy custom campaigns and rts games will find many of these games enjoyable.
I enjoy space engineers, there's just something so enjoyable about slowly building up your fleet designs and seeing them get better and better, then trying a survival game and realising it would take a week to build an escort... Then throwing them at a workshop-downloaded sport-warship (looks good, has features but hits below its tonnage and focuses on aesthetics). Testing them on a combat server can be fun also, although there's been some interesting times with having a 100 meter smart shell with component cluster explosives hit the admins flagship or "nothing can destroy this base"...then getting kicked...
i have just played through XCOM:Enemy Unknown, it's pretty addictive but has it's downers
i also recently play Satelite Reign, which is kinda hard. it's a hybrid between Commandos (stealth tactics) and syndicate wars (kill everything in your way). cyberpunk style always has a great ambiente.