Since U-23 teams have been introduced, national teams could also be introduced. I think it would add some flavor.

Many U20 players plays in U23 teams, another NT I don't think it will help with something.

I wouldn't mind this. More national teams means more engagement for the userbase. Players born in the off year between U20 teams are at a disadvantage for development into NT teams later on, this would remedy that a bit (though I suspect there will be a lot more 22 year olds in the tournament at least compared to the 19 years in the U20 tournament).