The only 2 events I've played this season used 50+3.
I feel there are many options to try out now that we've given 50+3 a good run, and players are a little unhappy with the time constraints.
As far as I see it, there are 3 easy options that can replace this format:
1. 60+3. This system would drag tournaments out even longer, thus not solving the "days take forever" problem, but at least solving the "dissatisfied feeling" problem. 10 extra minutes are greatly appreciated. Even 55+3 would be better.
2. Reintroduce tie-breaking procedures. One of the reasons 50+3 has been so frustrating is that a winner in game 3 has been unobtainable. I've seen many games end after 3 turns where one player has only 1-2 prizes left, and would win the next turn, or at least in the next couple turns. The system of "over 50% of prizes taken etc..." was a good tie-breaking procedure. With the new first turn rule, it would be difficult for fast decks to abuse this system like they could have in the past. A slower deck like e.g. Trevenant/Accelgor would probably feel just as comfortable playing in the 50+3 environment even if this restriction was put in place. I think this change would be met with raucous applause by the player base. This also doesn't aid the "days take forever" problem, but that might be a sacrifice we as players have to make if we want a "Best of 3" format. I see no way around that problem without...
3. Returning to single-game swiss. There are merits to this approach in the current format. One is that this system does away with the exceptionally long tournaments. That's probably the best reason to consider this option. Two would be that deck interactions seem to have become more complex since the new first turn rule was introduced, and fewer games are won on speed and power alone (the bane of the old Single-game Swiss system). This second reason is more of a "lesser of two evils" acknowledgement, as I am sure implementing this system would have players crying out "BS format, dead draw = tournament loss" and such. But players will always complain, reducing those complaints as much as possible is how to measure success.
One way to minimize problems with Single-game Swiss is to add more swiss rounds. Swiss+3 rounds cutting to Top 8 would still result in shorter tournaments, with a reasonable appeasement for the players that lose out on luck early. I'm sure players would be dissatisfied with the Top 8 cut, but they are already, so nothing lost there at least. The overall record is what players are looking at as an unfair cut right now, despite the Bo3 system. For instance, A 7-round tournament where 5-1-1's are missing top cut would become 10 rounds where 8-1-1 is more difficult to obtain. Using Sixprizes' Top Cut calculator, it reports that a 100-person tournament with 10 rounds of swiss would result in ~2.5 records of 7-3 making Top Cut. Given Ties, I imagine that would be reduced, but I bet you would exclude fewer equal records with this system.
Beyond these, there are many more options worth exploring. For instance, making Top Cut stretch deeper and giving Byes in the first round of Top Cut to top-placing swiss finishers. This would allow more players to play off in Top Cut, with fewer rounds of swiss, obtaining a similar overall length to an event. This would be a similar system to the single-elimination Top Cuts of VGC Regionals in 2011. Just cut until an even-numbered bracket is obtained. But you could design it such that... say... the Top 4 from swiss get a Bye, and you cut down to Top 12. This would let 12th-5th play a round, then re-introduce the Top 4 seeded appropriately, and you've only added 1 round of cut. It rewards strong swiss finishers and ensures that equivalent high-winning records aren't excluded. This would be a system that goes quite well with single-game swiss reintroductions. If long events are a concern, Swiss+2 rounds would be met with a better response if there was a smaller chance of whiffing cut when you're X-2 (though running simulations on the calculator, +3 rounds is highly preferable).
I trust TPCi and Play! Pokemon to do what's best for the player base in the future! I'm very happy to have seen a great improvement in the format and tournament structure this year overall; although I've been unable to play as often as I'd like. I think overall players at least appreciate Bo3 more than the former single-game swiss structure. But now players are more tired and exhausted at the end of the day, and reflect on the day with more negative emotion than they did with the single-game structure, creating a more negative tournament experience overall. For this assumed reason, I think solving the "long day" syndrome is the top priority, and am thus in favour of option #3 from my proposed list. But this is your guys' job, and you'll spend a lot more time thinking and considering this than someone like me! Looking forward to what you come up with for the future!