Sorry I didn't notice your answer, I was typing a new post in the meantime. Let's say the best 8k teams get invited.
It says 283395 games simulated in last 7 days. What is 4k games more? I don't know how the system works, but doesn't sound like this would put a big strain on servers. And the number of games would be decreasing in half every week anyway, down to one game in the final week of the season :)
Also, doesn't sound too hard to program. I think B3 already draws remaining teams by the world rank, if the total number of participants doesn't match with league/cup/past winners? So it shouldn't be that hard to seed the teams by the world rank? Schedule would be fixed, so it would probably require less resources than cup seedings?
Anyway, the main benefit of this would be it's massiveness. Not counting private leagues, teams can only test themselves against other nation teams in B3. And it's hard to qualify there :)
This would be like an intermediate achievement. It would give managers something to look forward to accomplish, it's not easy, but also not hard getting into top 8k teams.