I don't think the U19 NT would give any benefits as there would be mostly players with 1on1 skills or bigs with either ID or IS. So I don't think that would be a great idea. U23 might be a good idea, but it may be difficult for micronations due to overlapping with the players between U23 and NT. But in general, this might be good addition.
For the naturalization, I'm against the system you suggested as it would mean that all the nationalities at the transfer list are now changed from randomized draft system to "highest bidder gives the passport" -system and I really don't like that. If the naturalized players system would be implemented, I would say that there must be two conditions where first gives player a double-nationality (helps with economy) and second gives NT eligibility.
1) Player must have played over 200 league games (~9-10 seasons) in national league to get double-nationality (if player is sold to another team in the same country, earlier games still count)
2) If player gets the double-nationality and doesn't have any competitive NT games in his original home country, he is eligible to be selected to his new home country. However, player can be eligible for only one NT (eligibility will be locked when player plays his first competitive NT game).