I have seen a 1 star Gjall before I thought it was a normal one, but only realised as it was closing. Luckily some nearby Dverger helped me out with it.

More starred enemies either pre or post boss killing would be worth it anyway. More common starred trolls (excluding raids) as you progress through the bosses would be good too, however it works anyway.

Fenrings, Ulvs & Cultists would be better with starred variants too, but only if they get a huge loot buff overhaul to go with it.

But then again stars before bosses killed might be part of the new settings they were teasing a while back too, so there is that I guess. I'm easy either way.