Suggestion, Since we cant use HL:A assets, I was thinking that there should be a universal community texture pack. For commonly used textures in maps. Similar to what hl2 textures where to gmod. These textures would be generic. For example: Grass, Roads, Bricks, Concrete, Glass. Doing so will make the map fit each others style in a cohesive way like in gmod. Similar to what makes gm_construct look cohesive to rp_downtown.
On how this could be done: I think this can be similar to the DM game mode repo, but this time around its crowed sourced. Where Facepunch does the work and the community can help out and make suggestions on what needs to be in, because they will use it.