Dont use Reddit username or Facebook username or anything like that even if it could be cool. That way you dont have to worry even if some log file or error stack trace recorded your username.
Not really - a SQL Injektion would be naming my user user"; DROP TABLE USERS;-- or something like that.
My string is harmless but might scare web devs: How often do you see pseudocode like this?
log(INFO, "User "+user.userID+" logged on")
Usually this should print User @someone@example.com logged on as the variable content gets converted to a string. But if the variable is not a simple datatype like string, but a complex one, there might be errors. If you do the concatenation above with an object instead of a string, this would only print User [object Object] logged on. Now when the web dev sees this line he's going to try to find the Bug, where this variable gets set to the wrong value/type, when in reality it's just your carefully crafted user name.
If you open your Browser and hit F12, go to the console and type
console.log(""+{}) you get an Object {} converted to a string by joining it with another empty string.
Don't use "user" as your username. Use a personalized one, because it is much easier to identify and obviously it is a requirement in a multi user context.
If you share your logs pseudonymize them, but pick anything other than "user", because it makes it confusing especially when dealing with pam, ssh etc.
I don't think this is as much of an issue as people think it is, unless you're going to be sharing your logs with others it's not really going to be much of an issue, especially if it's not your real name. You might be thinking of some app or malicious program looking through them, instead you should be thinking of the real danger of an app or program opening and looking through Personal files on your computer, that's where the real juicy stuff is.