Some quick notes jotted down for how a theoretical campaign involving my two rainworld OCs, Relict and Inheritant would go. You'd switch between playing as Relict and Inheritant using the special button strategically depending on the situation (not written here because I ran out of room).
When playing as one, the other would gain SlugNPC AI- meaning your strategy would mainly be avoiding enemies as Inheritant while Relict piggybacks and acts as a normal pup, and switching to Relict as a risky offensive strategy or for getting out of a pinch (with a remix setting for friendly fire- careful with that axe, Relict).
When one is grabbed (and not killed) control would automatically switch over to the ungrabbed slugcat- but one being killed might as well be a loss since it'd lock in a bad ending for the campaign- and also just be really sad.
There'd also be built in jolly co-op support where one player could play as Inheritant and the other as Relict- which further cements my acceptance that this would never be a real modded campaign.
In terms of story, I'm still working on that- been in a massive rainworld mood lately so who knows, maybe I'll whip up a little something soon and finally get myself out of my writer's block.