Series Categories
A lot of games, sometimes smaller, sometimes larger, are spread out across a series. Games within that series share a lot of similarities to viewers and being able to browse by a series would be massively beneficial to discovery.
If I search for "Mega Man" and go to categories, I could see all the individual games; however, being able to see broader categories of "Classic Mega Man", "Mega Man X", or "Mega Man Battle Network" would make that search much easier.
This is not an idea to create whole new categories to stream to, nothing would change for streamers. This is on the twitch front end to display new categories that group games from the same series. This would show as a new option in the browse page for "individual games" or "series" when viewing categories. Any games that do not belong to a series would still be able to show up under that view (ex. Fortnite or Fall Guys).
Grouping games like this solves multiple issues by boosting smaller games in a large franchise, condensing search for an old series that has multiple games, and better filtering for what people are interested in without needing the extra effort of searching for tags.