Please, do not change the Username system.

I've spoken my Two cents. I'm going to do it again, because I'm not convinced you will listen to your user-base anyways, so I will take time out of someone's day to either respond or filter this message out.

Why are you targeting a non-issue? No, wait, I know why, so don't answer that. It's integration with a third party. The "Pomelo" test says that enough.
But to call this a way for users to express themselves is blatantly false advertising. You are enforcing a more restrictive system for simplicity's sake, which is still a bad idea. You are taking away customizability for users, not adding it. The removal of 7-bit ASCII-based languages also prevents foreign users from having names in their native languages. Congratulations on alienating foreign users.
The enforcement of simplicity also makes it easier for impersonators to make their way in and keep stolen accounts. You'll have an influx of bot farm accounts now to handle. This is not a fix, it's a problem, all to better streamline better with current social media platforms.
Not to mention, users are going to have to now deal with more inconvenient and harder-to-search usernames with xX__[Username]__Xx -esque modifiers, or inevitably have an influx of random friend requests from users who are trying to find one of the same 12 people with [GenericName] as their username with maybe a single letter variation that inevitably causes confusion.
If this was to make things easier for Developers or admins, you're lying. There is already a User ID system for across application bans and strikes, or in worse-case scenarios, pings should a username be too difficult to ping on its own. 
Plus, there were users who paid to have their digit code edited and changed. This system was so great that the actual number tags became part of the expression of an individual's account. I have not and intend never to change my Discord tag, not even the number code that I've actually come to memorize as a part of my username.
Perhaps I'm just an outlier, but I would rather not fight thousands of other users for the username "SecondUNIT" like I've had to on several other platforms. I do not trust the fact that my account is 7 years old, and may have earlier priority over other users. The fact that I could makes this feel like a disingenuous system as well.

So instead of targeting a non-issue and not talking to your new partner integrator, perhaps you could tell your team to focus on more pressing issues?

First off, It's in extremely poor taste not to make new features more easily accessible for your userbase to test. I should not need to enable developer mode every time I want to see what new mistake the Discord team is attempting to cook up simply so I can be prepared to question my dedication to your application and web service. I do not use Twitter (and I presume many people share the sentiment that it's not a place that is worth time anymore), and while I'm not a user who is maxed on channels, I know people who are and either cannot or will not add additional clutter to their allotted server space to make room for your developer discord. Make it a toggle feature instead. That way you can also avoid users trying to send concerned users a line of code that can manipulate their client. Just a thought given the 7 warning lines I get every time I open up the console window.

Second, actually add integration between the client-side discord and the help desk. The fact I was required to make an additional account, that also has discord integration, now means I have 2 discord accounts, one of which will never be used. If you intend on using it as an additional account to inflate your user numbers, know that either after this update fully rolls out or if it gets removed, I'll be deleting this account.

That will be all. I don't want to threaten my Nitro, but I will. I do not need it to continue to play with my friends, and if all else fails, there's always Skype or Teamspeak. I have stuck by Tumblr; I will hold onto my archaic internet practices.