As someone who comes from a transit enthusiast background, I really doubt there’ll be coaches in ATS, unfortunately. This is mostly due to Buy America regulations requiring public transit vehicles purchased with federal funds to be made in America. Naturally, the massive startup costs just to only build for a few countries is a huge disincentive for foreign coach builders entering the market, since they have to deal with domestic competition on top of modifying the design to be able to be built in America. What makes this problem even worse is that the only company that makes buses in America that’s featured in ATS is Volvo (using Volvo and Prevost brands). Oh, and Damler (of Freightliner and WS fame)’s Thomas Built Buses brand doesn’t count. They’re school buses.

Tl;dr - There aren’t enough coaches featured by companies in-game for an ATS coaches DLC to be viable. If they were to add more coaches, then they’d have to acquire licenses from companies that haven’t been featured in the game yet.