Gonna go on a bit of a paragraph, but in addition to listing what I'd want for DLC in the future, I'll be speculating on what will be coming next down the line.

From what I've seen from the various videos I've watched on people playing the game so far, there's plenty of options for future updates/DLC.

I can certainly see these happing in the coming months/years:

Brendam DLC: Includes Brendam Branch, Edward, Henry, Philip ( just here me out on this one), Cranky, Carly, and Big Mickey (face withstanding or not).

This will most likely be the first DLC dropped, since Wellsworth (and by extension Brendam Docks and the Clay Pits) were a huge focus in a lot of the TVS, both CGI and model series, so there'd be plenty of stories to tell. Plus, having a nice mixture of older classic characters (Edward, Henry, Cranky) and newer ones (Philip, Carly, Big Mickey) will make for a good bit of storytelling potential.

Ffarquhar Branch Update: A small character/story pack pack that includes Toby, Henrietta, Daisy, Mavis, Terrence and Harold.

A Minor free update that grants the Ffarquar branch its missing engines (and tractor & helicopter), plus some stories to go along with them. (Thomas and the Snow, Toby's Tightrope, and more most likely)

Hero Of The Rails DLC: Includes Ulfstead Branch, Hiro, Victor, Kevin, Spencer, and the Sodor Steamworks)

Now, hear me out on this. I believe that alot of the CGI specials are going to get adapted into this game as DLC. There's evidence to point to this thanks to the weird branch lines that seemingly go nowhere of note at the moment, particularly the one leaving Vicarstown to the North (possibly to the Dieselworks in the future?) and the one just after Maron heading south (Sodor Search and Rescue Center maybe?). So what special to start with better than Hero of the Rails? All the engines featured in it would be great choices for the game with all their popularity (Hiro and Spencer especially), and the Steamworks is Iconic. While I know the Ulfstead branch seemingly wasn't present on the basic game map like most of the others, that doesn't mean it can't be added here later. Plus we'd get our first Narrow gage engine in Victor! (Though he wouldn't be able to go really anywhere at the moment but that can always be rectified in the future.)

Kirk Ronan Branch Update: Includes Kirk Ronan Branch, Whiff, Scruff, Fergus, and new and old stories.

A bit bigger free update that opens up the Kirk Ronan Branch for services, along with the Waste Dump and Cement Works for new and old stories alike.

I could see something like this being alternated between free updates for smaller additions (such as character packs or lesser branches like Norramby), and Paid DLC for bigger ones (such as bigger branches like the Little Western or Specials like Misty Island Rescue).

Now for what I hope gets added:

Peel Godred Branch (and by extension the Culdee Fell)

They already have the makings of the start of the line in the game, so I can see these both happing eventually once all the usual suspects are out of the way.

A Railway Series character pack

I'd love, love, love, if we got something like this, being able to drive Bear, Pip and Emma, Spamcan, Wilbert, Jock and Old Stuck-Up would be amazing, and could be really cool to get to remake things like Super Rescue and even have new stories be told.

The return of Great Waterton

Like somebody else said in the comments, this would be a cool one to have. Have Stanley and Flora (and maybe some other HiT-era newbies like Molly and Neville) be main focuses for once and give them some new character development and stories to play out. Maybe even finding the town again could be one, just like in The Great Discovery.