In both D3 and PoE, I've always played one character per season. I don't really do "alts".
Knowing this, I'm planning to do the same in D4. I've only leveled Rogue to lvl 100 since launch, but haven't even created other characters. I'll just spend the first few seasons trying out the different classes. Probably going Sorc for S1, likely starting out with Lightning but time will tell if I stick to it or just try out multiple different builds.
I always played one character per season. There were a few times I rolled alts in a season and they were always a massive pain to gear up. It was never worth it.
Now, we don't really know what the first season is going to be like. Rolling an alt might not be that big of a deal, who knows. But I have a hard time imagining it not being a waste of time.