Maybe so, maybe so. I'm thinking of taking a look at CS:S maps and assuming I can find a suitable map, I'll port another campaign. While Militia doesn't really seem like one, Train and Nuke are possible candidates...

Whichever maps he considers, he could still tie all of them into this one single campaign.
Think about it, makes sense, if length is that much of a concern...
As well as seriously considering porting more Counter-Strike maps.
Just see what happens combining them altogether into this current one first?
You'd cover all the bases.
Oh, and whatever ones that might not be campaign worthy could be for survival mode.
Someone did that with that Ravenholm campaign... the part with Father Grigori.

As Ryan quoted above, I am planning on making another one sometime in the future (after the current movie project is concluded). I took a look at Nuke and Train yesterday, and so far Train seems like the best candidate in terms of layout. I'm more than happy to take suggestions, though, but the map to port should meet the following criteria:

• The original map MUST be by Valve, ie. a "vanilla map".
  
• Generally speaking, the more detailed it is, the better. Good examples include cs_office (duh), de_nuke and de_inferno, while maps like de_cobble and cs_havana wouldn't make for a very exciting campaign.
  
• Additionally, the more content the map shares with Left 4 Dead series content, the better. I'm not very fond of including CS:S only assets as custom content, and considering how it can increase filesize, I doubt most people wouldn't really like it either. This can be pretty hard to
  notice without mapping experience, though, so don't worry too much about this particular criteria.

About content and file size... well, I can't speak for everyone, but over on "Game Maps" I didn't see too many complaints about Half Life 2 maps ported with the assets (though to be fair, some authors had those assets as separate vpk files). Although the porter of the Ravenholm campaign managed to successfully find and include only what Ravenholm needed in his vpk. City 17 appeared a complete recreation from scratch. Anyway, I don't see this as a concern with you, though, the point was if the players really want the content, they'll download it... content would be separate if it exceeds the limitations of the vpk format itself ("Absolute Zero" campaign).

Consider what I mentioned earlier about just adding the new maps into this campaign, though, it might work out in the long run and then you won't have to worry about short length any more. maybe it'd even be more convenient for you to just keep everything in this single campaign.

What about overpass? It is one of my favourite csgo maps. If it would be to short for a campaign you could add a access to the city or it would be a survival map.

idk or my mods crashing or ur maps?

A very common issue is that many mods that alter textures (usually HD versions of models) cause crashing on custom maps. Disable those and try again.

that might be true, but in my experiences they just carry over into others and stand out.
for example, there's a Parish texture mod, I forget the name of it...
and it carries over into other maps and just sometimes looks awkward.