Well, maybe this will help: Sega have announced three updates to come in 2023, all of which will be free, and the last of which will add new playable characters and story.The roadmap was laid out on Twitter:
The first 2023 update will bring a photo mode, a new challenge mode, and a juke box of (I assume) in-game music. The second will bring new Koco, which are one of Frontiers' collectibles, as well as an Open Zone challenge and, uh, "Sonic's Birthday". Apparently Sonic's birthday is June 23rd - which is only six days after mine, in case you want to also add my birthday to the game, Sega.
Update 3 is the most interesting of the bunch, as it will add new story missions and four new playable characters. Those characters are Tails, Knuckles, Amy and [opens Sonic wiki] Sage, an AI girl created by Dr. Eggman and first introduced as a non-playable character in Sonic Frontiers.
Sega confirmed in a followup tweet that the updates were "all free btw".
I've not played Frontiers, but Ed's review did note that it was "a great foundation for a sequel," as it had potential "which only needs a bit of fine-tuning to get Sonic from simple to super." Perhaps these free updates will provide that fine-tuning rather than a sequel.