They are more easy to build, they can have much more exciting and picturesque environments; and you can put much more creativity into them: I am talking over a fictional route. At the moment, we have only two smaller ones: The Holiday Express of course (which was a little nice and funny idea) and - don't forget - the Training Center. I'm not talking about making it the new standard; but mixed with the (more and more upcoming (?)) steam locos and maybe a more (stereotype) fairytale/fantasy environment: at least one "creative/art route" could work very great. In contrast to the already existing ones, I am talking about a whole route; not only a place with a few rails; - means: you can drive from one end to the other one in around 45 - 60 min (similar to the most other existing (not fictional) routes); also with stations and interessting, interactive points. The route could also have some realy interessting details and switch much more rapid ins aspects like: a lot of rails into a single-lane scenery (because you don't need to seek for these things in real existing routes). Which - overall - could also include much more various scenarios and time table services. (Thx for the theme pic @streamcommunity)
Not gonna buy the Holiday Express, and I only use the Training Center for the tutorials. Fantasy routes aren't my cup of tea. The only 'fictional' routes I'd be up for would be realistic route suggestions that never came to be. (Like the North Somserset Railway DCL of TSC).
I also have a suitable idea for a fictional route like HDX. It's a garden railroad, so a couple of gnomes would also go well as decoration. I put that together at the end of last year when HDX came out and it was said in the live stream that more such projects could be realized in the future. I would also leave fantasy routes in the area of toy model trains, because routes for real trains should continue to be based on real routes. TSW should remain a real railway simulator and not a construction and management simulation like Transport Fever 2.
TBH, the only fictious route that could work would be one set on Sodor. With the inspiration and reference material coming from the Railway Series books
This suggestion ain't gonna fly for those that want real accurate routes. Not to mention that the Training Center is based on an actual location, and the Holiday Express is just that, a Holiday Special. Best for the resources to be useful on making existing routes (or as close as possible), rather than making something that's very random and fictional, whether if it's for Steam, Diesel, or even Electric. It's just not worth it for TSW.
Pretty sure it's also to test the waters. If Holiday Express sells well, I'm sure we can expect similar releases in the future.
For kiddies level of fun for the next special DLC, still doesn't mean that everyone will like it however. Plus, there's already a few holiday decorations as layovers in training center anyways. So I doubt they would make another holiday dlc at this point.
The DTG devs have said so many times that an editor is NOT coming to the game because there are hurdles with the game based in the Unreal Engine, licensing and more.
The only fictitious route would be one that was planned but was never built (e.g. Glasgow Airport Link)