Since I can't make maps myself well enough to release I decided to post some suggestions here and see if anyone is gonna attempt any of these. All of my ideas come from Metal Gear Solid though, so if you are to attempt them you would need to have played the series (MGS2 & MGS3 at least, as that's where these ideas of mine come from).
My first suggestion comes from Metal Gear Solid 3. Remember that hill you had to climb? One where had helicopters. This map is also on MGO(Metal Gear Online, featured in Metal Gear Solid 3 Subsistence). That map can be a perfect domination map or can even be CTF. In a way, MGS3 & HL2CTF are a bit similar. Both have RPGs, both have sniper rifles, both have grenades(obviously), and both have different machine guns (AK47 and M16 for MGS3, SMG and AR2 for HL2). In MGO, the map is quite medium to large, it is an open map (means outdoors) and I guess it can handle ~16 players (however limit for server was 8 players in MGO).
My other suggestions comes from MGS2. Tanker can be remade I'd think, since it really was not a big map in first place. It can prolly fit on 1 HL2 map. Another idea/suggestion is from second chapter of MGS2, one of BIg Shell struts (or multiple?). I think it can work, struts in MGS2 were not large. At least 1 of struts has a helipad (so an open area) so the map can be a mix of both, indoors and outdoors.
That's all the ideas/suggestions I got for now. I wish I could make them into maps but I am still trying to learn dang Hammer editor. Once again, those are just some suggestions I have from MGS2 & MGS3 I am sure if you look through those games you can probably find a lot more ideas (especially from MGS3 since it featured online mode).
If no one is willing to make/try to make any of those maps maybe I can try...in like 2 years lol.
PS: I was not 100% sure where to make this post so I just tossed it here since most of maps discussions take place here...