The first step to getting Intermission started is to set up some rosters for each faction. Please come and comment on the current draft.
Each faction has two sets of Champions listed: Primary and Allied. Primary Champions are always available. Allied Champions are almost always available, the exception being when facing their Primary faction. For example, Poppy is Primary for Bandle City and Allied for Demacia; Demacia gets her in all matches except those against Bandle City.
As for Champion selection rationale, I’ve erred on the side of inclusiveness. I thought about even including all of the arc-specific associations, but decided that would make things a little too zany.
There’s a discussion thread on the forum, as well.