There will be a new user API; but it is not yet available. In the meantime, I suggest writing a separate class to include in your code that you can implement this in now, then when the new API is available, your class can serve as a wrapper to allow your interim API to interact with the new API.
Most likely, a user object will be available and the methods of the user object would be used to manage the data, e.g.
...
if (is_object($modx->user) && $modx->user->isAuthenticated()) {
$modx->setPlaceholder('username', $modx->user->getUsername());
}
...
or
...
$user= new cmsUser();
$user->setUsername($name);
$user->setPassword($pwd);
if ($user->save()) { ... }
...
but again, none of this has been set in stone as of yet, and the end API may differ from what I’m describing here.
I’ll talk more about the new user architecture after some more decisions are made.