Hmm. I don't think that Ethan is the world dominating type. He seems to be more of the pop up and cause some mischief and then leave. I'm not sure if there is a season where he would fit in as the big bad. Part of what makes his character interesting is his relationship to Giles. So coming at it from that point, I think season 2 would have been his shot. But I would not trade Angelus, Spike and Dru for anyone.
Since he gets captured by the Initiative in season 4, he couldn't have been the big bad for that season. It's not until the Initiative gets dis-banned that maybe we could have seen him come back and it's a shame that he didn't. But I'm curious to see what other scenarios ppl could come up with.
The only other thing I could think of is maybe if he was like the leader of the Trio. Having them build things and trying to take out the slayer. Maybe a Willow/Ethan magic show down at the end. Idk