I think this is a major detail. I get why you take the prefix DW-, but they should be able to choose their own name after the DW- prefix. I bet this will be very important for people... I mean who of us here does wan to post as "398623089723#ztz" ?? New users will likely not use this the way they could... so I say good initiative and plan, the DW prefix is fine but people should be able t chose their own username and I believe this is very important.
You should also make clear how important the keys are and that they should securely save it and print out etc...