Uhhhhhhmmmmm! eeeehh?? Meaby the easiest way is it to make a 2.8.5Assist.
Ore ask the coop guys AAOTTS if they can make ore turn all ther normal maps into 2.5 assist maps .>>>>
I think i try that whit permission of you guys from Assist.!!!! becose one mail is easy to do. 
There's a certain amount of custom maps that are worthy but you should keep in mind that a lot of the Coop maps are made with a focus on cooperative gameplay, so many are physically way too large to ensure a good gameplay and ease of exploring/patrolling all over the map, whle others are made with different gameplay mechanics in mind than PvP gameplay. Of course i'm talking about PvP because it's the only way you can play those maps in a 2.5 client, as AI is not a viable option for plenty of functional reasons.
A practical way to let everyone convert their own map to 2.5 might be Eliz's tool, but of course that also mean using 2.8.5 resources, and i mean all of them, because they are not organized in a proper way for us to exclude what is already included in 2.5 and keep newer assets, otherwise it would have been a much easier process (in the end it's a matter of space as 2.8.5 materials are probably worthy for at least 1gb and a half, and they would grant you just the ability to use the materials with no further newer functionality).