Ok, i started criticizing this, but idk if one should overstate this problem of 5x5 being too small... I know that flipster and FairyTail got bored of 5x5 too and maybe there's others but that's just a handful players (although it's the players I value especially high). With a playerbase this slim one might as well keep using 5x5 since there isn't much development going on, it's just that for those few people it won't be much fun anymore, the majority is hardly anywhere near mastering 5x5 though and 7x7 is probably much more unfriendly to beginners than 5x5.
Well, on to the scenario at hand: There's no way 5x5 would stay default if DnB became as popular as chess, but I'm going even further and say that even 7x7 might not suffice if it came to that. Let me provide some historical background as to how 5x5 became as "solved" as it is right now. I'd say there was like 3 major metagame-periods on littlegolem: The first one didn't really have anything like opening theory, it was mostly just figuring out how the game works and the player michael did very well there. Then the second one was a huge flipster hype who came up with 1.f3 2.h9 and the strategys surrounding it and that was a thing until very recently with sghsal at it's peak. Okay, and then 7ics appeared out of nowhere who was pretty strong but learned an entirely different metagame somewhere else and he was convincing others that 1.f3 was actually a pretty bad move all along and that's where we're now. So instead of 1.f3, 1.f5 is now the move that has gained increasing popularity, but there's an issue with 1.f5: it's too strong. The 1.f3 2.h9 period was really great since the second player had really good chances there and the first player advantage seemed very unimportant so it was a lot fun playing, but 1.f5 doesn't have that. Well, on with the main point... just a few strong players managed to break 5x5 down to what it is now. But all these players who explored it put together wouldn't even matter in a community as big as the one for chess. There would be several thousands of players stronger than I and they would destroy 5x5 during the blink of an eye.
Alright, on with 7x7. That board looks ridiculously huge compared to 5x5 when you take a look at it at first, but I mentioned that even 7x7 might be too small for a community with thousands of strong players and I'm saying so for a reason. Let me show you one game: http://www.trmph.com/dnb/game/lg-1668278
This is my last game I've played on lg and the point where I thought I was winning for sure was no later than move 20. There is on 5x5 positions with 20 lines played where you just can't tell who's winning. Games on 5x5 that decide around move 15 are in fact not a rare occurrence, so there's maybe like a 5 move difference between the decisive point of 5x5 and 7x7. Alright, and then there was this issue about the first moves on 7x7 being random. I want you to take another look at that 7x7 game and compare the position from move 20 with the position from move 8. The thing is that this structure from move 20 where I knew that I was winning was already predetermined at move 8, the whole structure it all exists in move 8 already: The bottom right part that is cut off, the top and bottom left corners looking similar and the center part that is cut in two at move 20 but was already expected to be cut in move 8. This is showing that early setups are possible on 7x7. Now let's go back even further and take a look at move 7. Move 7 was cutting the right and then move 8 was saying "no, don't do that". Now the question is: Did anything change from move 6 to move 8? Some might find themselves answering "no" so one could argue that the position in move 20 was a natural result of the position in move 6. Move 6 is seriously early as hell, but I'm saying the structure for the entire game was already there, but now I'm saying that the structural concept from the position after move 6 was intentionally build by me as well, starting with move 2. Nothing is random about the early moves in 7x7 and I'm sure that a huge community would be able to grasp the 7x7 openings just like we managed to understand 5x5 so even 7x7 might not last very long in a chess-like community.
Sean, you asked me if something could bring back the excitement for me. I see multiple options here: 1.Modifing 5x5 maybe with something like the swap rule in order to increase the chances for the second player, that however would turn the 5x5 game even shorter and might only satisfy temporarily. 2.Moving to a different board size (7x7), however that may be problem for those trying to learn the game and I mentioned that it might not be worth it doing so for the sake of just a handful players.