Показывать полное имя только в определённых категориях

[Lilly]

there is a similar feature request to your OP already here:

Restrict exposure of full name to certain groups

Background We are trying to set up our site to strike a balance between open access and privacy concerns: The site is public overall; anonymous users can browse, and search engines can index it. Usernames are public. Real names (full name) and profiles are considered semi-public; they should only be exposed to other community members (not to webcrawlers or anonymous lurkers). Email addresses are considered completely private; only admins should have access to those (and access is monitored). …

from your use-case description in your second post, I think this would require a custom plugin.

@newbit1337

A user registers with Username + Full name.
When creating a topic, they can choose if they want to show their username, or Full name.
Would this easily be possible without the security aspect?