This is a theoretical concept for a game that may be run for a smaller group over a small stretch of time.
The idea behind the Gaiden concept would be short-run games (one to three months) that explore an event or place that has not previously been well-explored by most players. It is an opportunity for players to play classes or races they would otherwise not play in a longer-running game, as well as experience things they would not normally be able to given the nature of standard Trinity runs.
Given their more focused nature on places and times, Gaidens will typically have a more restrictive list of classes and races to choose from. Most Gaidens will begin at lower levels, and will typically not break 10th; this is an exploration of various side-stories throughout the setting, not massive world-changing events, and as such the characters are not necessarily "relevant" to the greater going-ons throughout the setting's history.
Due to the shorter run and space constraints, each Gaiden will be limited in players - I'm thinking between three and five. This is to prevent the games from getting too out-of-hand and losing focus. For now, at least, each game will also be relatively plot-driven: unlike other Trinity games, however, players in Gaidens will get to decide a good deal about when, where, and what they're playing. While it would perhaps be possible to run a more sandbox Gaiden, I think the dedicated time constraint will lead to most of them being on rather constrained rails.
Due to the side-story nature of the Gaidens, they will exist outside of the "standard" Trinity season system. Instead, each Gaiden will simply have a number.