It looks like Sonic Frontiers will have DLC, based on this promo

destructoid.com:

Sega selling add-on content isn’t too surprising, but we may have our first soft confirmation of Sonic Frontiers DLC thanks to a retail listing.

The details come via a Taiwan game seller, who opted to put up an international promo image to incentivize pre-orders (as spotted by VGC). On it, in addition to bonus content, is an ad for future DLC that is “in development.” The listing also showcases a November 15, 2022 release date, which could still be a placeholder or subject to change. Thus far, Sega has not provided an official date, but given that it’s going to be seen at several tradeshows in the next few months, an announcement could be imminent.

DLC for recent (comparatively) 3D Sonic games has been very muted, so my expectations are a little low. Sonic Lost World had a few free Nintendo crossover lands, and Sonic Forces hosted “Episode Shadow” and “Super Sonic” side stories. But as far as major expansions go, things are light. Maybe Sonic Frontiers will change that, and given the nature of the mysterious Frontiers universe itself, it would be an easy candidate for whisking players away to far off lands. The Cyber Space aesthetic is already kind of doing that.