user account url without prefix

Question

I try to create a website contain user account system,
the user profile page, url path is domain.com/:username. I don't want to use prefix like domain.com/user/:username .

So I have to find strings avoid user register,

1. My website other url path first slot is domain.com/filter/, domain.com/search/, domain.com/p/, ... I will add these.

2. I read about other website doc like google facebook .. e.g. facebook are specific list can't contain generic terms or extensions (ex: .com, .net).
   is there any other strategy (keyword not allow), suggestion I should know and use to avoid future problem?