A number of publishers are unveiling "content" at Gamescom, which could be anything from new vague trailers, to DLC, to new microtransactions. But Sega for sure has something big planned, as they used the phrase "all-new AAA game" in an email to Destructoid in regards to their Gamescom appearance.
The publisher also plans on showcasing Catherine: Full Body, Two Point Hospital,and the Sega Genesis/Mega Drive Mini. The former will offer a demo with cutscenes, Two Point is a full runthrough of the first chapter "for the first time on consoles," and "all 42 games" for the microconsole will be playable with a "giant fully functioning controller."
Sega has such a long and storied history and such a treasure trove of IPs that it's impossible to accurately guess what this mystery project might be. The phrasing of "AAA" is curious though, and remember: Sega has access to any number of studios they've acquired over the years, so this could very well end up being something like a PC strategy title and not a new Sonic game.