UPDATE: SEGA has later uploaded the theme via the official Sonic YouTube channel! Original story follows below.
The SXSW Sonic panel ended with a sneak preview of the Sonic Forces theme. The theme was composed by Tomoya Ohtani, who served as Sound Director for Sonic 2006, Sonic Unleashed and Sonic Colors. While this version of the theme is strictly instrumental, the final version of the theme will have vocals.
Check out the theme song in the embedded video below, courtesy of Blue Paradox: