You can upload a VPK to the workshop with new tilegen templates. However, to my knowledge, users will need to extract the files from this VPK file in order to use them. You can also put a link to an archive with all the content into your workshop item description.

I remember seeing one or two custom themes for tilegen. However they were pretty basic comparing to what is shipped with Alien Swarm, with clean walls and a few props.

Check out moddb and gamebanana websites, you may find them there

I'm thinking of chopping up parts of levels so they can be snapped together.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=919472916
I took:
...\Steam\steamapps\common\Alien Swarm Reactive Drop\sdk_content\mapsrc\ExampleAddon\maps\tutorial\example_map_1.vmf

...opened in Hammer;
removed the func_brush structure_seal,
moved lower-left -from top- of the piece to (0,0),
removed exit trigger & scripts, and then sized it to match the 256x256 gridspace in TileGen.

I need to become a little more familiar with the how-tos of detailing rooms and the triggers on different maps.

Saw the "Alien Swarm Director Mod," that looks neat. This is just self-reminder to checkout how new tiles work with that sometime, and if there is version for ASRD.