I only chose the word doughnut because I'm not creative.
But at least you understand a little of what I'm trying to say.
There's your problem. In fact, that's the
only problem. As long as people are creative, there isn't a problem. It's all up to you. Not creative? Not a problem.
Get creative! There's nothing that's stopping you- if someone else has a name you wanted already, who cares? One-up them and make something cooler.
The only problem with Steam users using simple usernames is that lazy people who want one-word names can't take 'em all. Even then Blockland probably isn't going to be so popular that absolutely
every single name that can be had is used up. There's really no issue here at all.