> We reserved a number of team names for known tech companies

I think this demonstrates the problem of creating completely new namespace from scratch. One way to solve that would be to couple the team namespace to DNS namespace; to claim a team name you'd need to demonstrate the control of corresponding DNS name, same way as you do for DV certificates. Of course I can understand that you'd want to support teams for organizations/groups that might not have their own domains, but you could support such cases by creating a "pseudo-TLD" (or just straight up buy your own TLD), for example .keybase.