Yeah I think of corruption in a game like this along the lines of cranking up the sex, cause most of the corrupted characters are sex obsessed where it starts being an issue, even in the sex world, and would ramp up their traits. Maybe sometimes they'd stop being companions due to issues related to their corruption. So off the top of my head, thinking of the three companions I know best:
Cait - She's already the sex positive cat ready to go, so if you corrupt her she'd just become full free use. Basically hers is easiest, cause she'd just go through Penny's bimbo path in TITs. Cait gets a level of booba one more cup from her current, maybe she grows a dick, and now she's so free use she's not a companion any more, but you can still haul her around, cause she's just a horny toy. Or if they don't want to reuse a similar story, make her more slutty, but now instead of being friendly and inclusive she's a mean size queen and likes to shame or dominate people. And maybe she grows a dick.
Brint - He's already huge in most ways, so maybe he gets a second dick and bigger balls, cause he wants to keep fucking even more. And now you have to have a bigger dick than him, or out fuck or outfight him. If you lose he leaves, and now you have a sidequest to get him back, and if you beat him, he'll follow you again.
Etheryn - She's kind of easy since you can make her confident or submissive. If she's confident she'll start acting more like her sister and becomes a sex despot demon. If she's submissive she's not full free use, but will now just do whatever you tell her, and you can transform her more for your preferences or her humiliation.