Just wanted to share my thoughts on my ideal format: I believe 3 SV/SS/SM/ORAS/BW/DPP/ADV/BO3 is the best way to go about this PUPL. As much as I appreciate the enthusiasm for tiers below ADV, I believe that the BO3 format showcases a lot more talent and hype than its being given credit right now. SS and SM both experienced a lot of development through PUBD and, while there's always worry regarding reusing, I believe that seeing the best players/pilots against each other is always gonna result in high quality, high stakes and extremely entertaining games every week. It also serves as a midground to those that want more SV, though that's more of a secondary effect rather than the point of the slot. I think letting go of what's canonically resulted in some of the highest quality games across generations in the tour that's focused not only on CG, but also on its very important previous gens deserves to be showcased at this level at least till the next generation forces us to make a new decision regarding the future of BO3.
In regards to GSC, I appreciate the support that the tier has gotten and I think that it will be a lot more exciting coming next PUBD, but I do not think that it should have a spot in PUPL at this point in time. When we started planning out PUBD, one of our main concerns was being able to fill in the RBY and GSC slots, we decided that 6 teams instead of 8 should be safe and that we might extend to 8 if there were enough decent signups for all tiers, this did not happen and GSC ended up being the slot with the most difficult pool to fill in. Some of names mentioned earlier that would be interested in playing GSC would also be considered by managers to slot into a different tier that isn't GSC, making the pool significantly shallower.
On the point of ADV having a smaller pool than GSC, even if that was true (I have no idea if that is the case), ADV has had a lot of development in recent years and has become a beloved metagame that managed to succeed last PUPL. This does not mean that we shouldn't extend our sights further and ignore GSC and RBY, or that GSC hasn't had relevant breakthroughs and been successful in its own right, but as it stands, removing ADV is out of the equation, due to the amount of current available slots and the success of Bo3 makes the removal of it equally unlikely.
It also goes without saying that extending past 10 slots would result in a lot of technical headaches, pools that lack more consistent players due to them being spread around different tiers (an issue that already happened in PUBD) and much bigger question marks in regards to the extra slot that would be necessary to fill in to complete the 12, and the dilution of the point of the tournament compared to PUBD.
TLDR: Read, lazy!