There's another alternative -- Your app is not your data, so there's no reason why you can't put some stuff literally at the top level. A Phoenix app already has two, as in your case, there's Rpg and RpgWeb. Nothing preventing you from adding to that top-level hierarchy for stuff that may be neither -- like, "Accounts" or "Billing" (ie. stuff that isn't really part of the RPG itself). That might throw a few people who will automatically go looking under Rpg, but it can help to keep those responsibilities separate.
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There's another alternative -- Your app is not your data, so there's no reason why you can't put some stuff literally at the top level. A Phoenix app already has two, as in your case, there's Rpg and RpgWeb. Nothing preventing you from adding to that top-level hierarchy for stuff that may be neither -- like, "Accounts" or "Billing" (ie. stuff that isn't really part of the RPG itself). That might throw a few people who will automatically go looking under Rpg, but it can help to keep those responsibilities separate.