I agree with you, but I think Ivy currently has a better potential for that role than Minerva. If Eva wrote Minera with a different motivation, I could see it. Like for example writing Minerva as a more dom character blackmailing Ian for sex, just make her clearly and unapologetically evil. But she chose to write her as a little bitch to awkwardly express Ian turns her on.
My take on it is that, as others have suggested with various storylines, Eva started with a clearer idea of Minerva as a character and that's gotten hazier as time has gone on. You can spin it positive and say she's now more "complex," or spin it negative and say she's been "muddled" and I think both are valid, and I don't really have a dog in that fight.
In the early part of the story, she's a very clear threat to Ian, with his job, and with the pitch for his novel to Seymour. The first time Ian can have sex with her, which is a very slow burn and doesn't happen until (I think?) at least a half-dozen chapters in, it's pretty much just hate sex. You can shade Ian's dialogue in different ways but her responses are pretty similar regardless: she despises him, even though he turns her on.
But I agree with you Minerva gets bitchy/whiny and awkward pretty quickly after that. Ian can quickly dominate and insult her if the player wants, and it's more hate sex. I personally like the route where, if Ian treats her well (in between screwing her brains out) she comes to respect him and actually open up to him about her life, but that does also make her far less of a threat to him - especially after Ian has switched jobs, and if he's gotten his pitch to Seymour one way or another (personally, through Lena, or through Holly). It's also possible Eva always meant for Minerva to be neutralized as a threat, in order for Ian to be brought into Seymour's orbit; we'll see. It's also possible, depending on player choices, that Minerva is still a threat to Ian winning the book contest because she's on the judging committee; we'll see.
I don't think I'd agree Ivy has the potential to be a serious threat to Ian. Sure, she outs Ian as cheating with Cindy to Lena (
if a player's Ian is cheating) but that seems to have a lot more to do with Ivy's anger towards Cindy over the Wildcats contract. If I remember correctly, the extent of Ivy being antagonistic to Ian without the Cindy situation is a fairly mild "Well, you seem to be sexing this Ian guy a lot, Lena, are you really sure you don't want to play the field?" (I'm paraphrasing, but I think that's pretty much it, even if Ivy and Lena do their own horizontal mambo.) And if Ian is with Holly, I don't think Ivy notices him much at all, partly because in that scenario, Ivy never gets a chance to corrupt Holly and the most Holly is to her is this shy nerd at the gym Lena is friends with. (Someone can correct me if I'm wrong on any of that.)
I also think part of why Ian doesn't have a strong enemy is they're either waiting in the wings (I'm thinking of Seymour and the book contest, and/or Wade and possibly also Perry depending on how the affair with Cindy shakes out) or it's simply that Eva doesn't have as much story for Ian as she thought she would have. Somewhere in the Q&As she says one protagonist is easier to write for than the other, and she doesn't name names, but looking at how much Lena content vs. Ian content there is in chapter 11, it seems obvious which one Eva is more interested in lately.