You can look at the roadmap for future developments under the Dovetail discussion board and also the announcement board for the dates of releases.
I think on the 8th August go to Dovetail Live and you will see a Roadmap On YouTube and a Website with the roadmap on it
At the moment there's not really much known. There are no in house DTG new route DLC's active on the roadmap, as being discussed in the General forum more than likely because "something big" is coming in a couple of months time. The only confirmed (UK) route is the one being made by Just Trains, 99% thought to be Blackpool to Preston possibly going on to Ormskirk. ATS are purported to be working on a route but nothing is known about when or where it is set, or when it's likely to release. Union Workshop were said to be working on a route but again no details about location or if the project is even still live. There are no announced third party routes from the other associates - Rivet, TSG or SHG.
Don't think tsg would regularly make routes cuz it's a very small team. But they are probably helping with dtg on all their German content and work still ongoing on the 420
At the start of the month on a Tuesday (if they keep releasing on a Tuesday) will be the roadmap, stated by the others already, but some months don't always have a roadmap release at the very start for example this month so it'll be the following Tuesday. This will give an insight on what's next to come, what's being developed and what may be in the thinking process. Another better way to find and gather information is the roadmap streams on youtube where matt and JD dive into the roadmap and explain and give extra information that could be really helpful.
It was more wishful thinking, if they could bring the quality of Nidd to bear on another project, I’d be in there on Day One!