I quite like the idea.
I think the qualification for the Euro's part of it might be a bit messy though.
If i understand it correctly, you'll have 20 teams qualifying directly from the qualification groups, plus 4 teams through the play-offs.
The teams entering the play-offs are decided by who wins their Nations League group, which is fair enough.
But then it get's a bit weird for me, the play-off format is basically a mini-knockout tournament of the group winners from each Nations league. So in the end you'll have a team from each league qualify, including the shit league (league D).
Which is fine, but then it gets confusing, because a lot of the winners of the league group will be qualifying automatically anyway, so their spots will be going to next best ranked teams from their league. But considering League A is filled with the best teams, it's pretty likely that they'll all qualify without needing a play-off, so then those play-off spots will allocated to the best ranked teams overall, from the other leagues.
One drawback i can see, is in some circumstances it's basically going to be beneficial to get relegated to the league below, so say Scotland lose their first three group games and have no chance of finishing top and getting a play-off spot. If they got relegated to league D, populated by all the shit teams, it's actually gives a better chance to qualify for the Euros the next year round, because the lower league is still guaranteed a qualification spot for the Euros, you just have to beat all the other shit teams.