No disrespect intended towards anyone, but I'm of the opinion that all of the "Primarch switches sides" ideas would be pretty much a slap to the face for players on both sides, regardless of said switching being towards the Imperium or towards Chaos. One side would lose a Primarch they could've had, and the other gets a Primarch in place of one of the others they could've had and actually wanted. I don't know, it just seems silly to me.
As fun of a brain or story exercise as it is to think about I totally agree with this. I get rustled as it is when thinking about wulfen Russ so if he suddenly turned for reasons I’d be really sad. I wouldn’t want that for anyone.
I wouldn't think Wulfenruss needs to be a bad guy. Mental exercise: the Wolves always knew their role was to be the Emperor's executioners per "Burning of Prospero" so if he turned feral looking in service to the Allfather I think he'd most likely embrace whatever his wyrd led him to. I don't want him to turn feral because they'd make him look like the Wulfen models with their dog legs that wouldn't support the weight of a biped. GW's "bigger is better" mentality makes me afraid that would be the art direction but I hope I'm pleasantly surprised.
With respect to art direction the 40k Robute is more ornate than his 30k model and Ultramarines are the GW box art chapter of choice so it's logical he was the first new primarch model. Which loyal primarch's 40k armor and weapons would look the coolest? Maybe that's which model GW would release next? They're a mini company first and story/game company second. I think the Lion's 40k armor would be the most ornate and detailed.
I think you missed his point a bit. It wasn't that Wulfen Russ would have to switch sides, but more 'I already hate the idea of them doing this (comparatively less extreme) thing to 'my guy', so having him switch sides would be even worse. Therefore nobody should have to go through that'.
But on topic. It's very 'watch the world burn', but a part of me wants Sanguinius and Ferrus to return. No fancy gets outs like Ascended Sanguinor or LoTD Ferrus, 2 whole body resurrections. Why? So much of the recent fluff has been tier imo, I'm morbidly curious to see both how the studio would do it, and how the community would react.