I don't think it's a practical suggestion. What's the main aim of this suggestion? What are you trying to solve?
From your first post it sounds like you're not happy with the people elected into the U21. But then you give them power to select their own assistant coaches, which (even worse) is optional. Doesn't sound like it would change a thing. Your new NT coach doesn't write in forums, and it sounds like he doesn't even set lineups some times, why would he even elect an assistant who will write forum posts for him?
The "different opinion" thing won't work either. What happens if the NT coach does not agree with the assistant? Will the assistant be able to force his way by sending in the lineup 16 minutes before game time?
Not sure if you're aware, but some NTs have scouts, some communities have open tactical discussions on their NT matches. All these are already happening, so there's really no need for an optional assistant coach feature. If your NT coach doesn't already have an assistant or scout working with him, or if he's not already discussing with another user on how to play the next game, chances are he won't have anyone to fill that slot as well.
Also, for example new managers like me can become assistant coaches for example and to see how exactly things happen in the national teams and such managers then will be prepaired to lead the national teams.
You can already do this now. Approach the NT/U21 manager, ask if they need a scout. If they don't, try approaching NT managers of other countries. Plus, if your local NT/U21 doesn't have match reports, write for them. It'll put you in the spotlight, increasing your chance to win an election in the future. This is probably the best way to get NT exposure as a new manager.