If the slots were actually increased, I feel like some significant balance changes would need to be made so bosses don't have a hard time dealing with 31 players. For instance, bosses would need to have increased damage, and some bosses would need to have increased health (e.g Zerstorer, duo bosses), and I'm guessing some changes to weapons would have to be made. So this task isn't as simple as it sounds.
Honestly though, I wouldn't mind this. EU FF2 server is full most of the time nowadays, with people struggling to join, so increasing slots would be an ideal solution for this problem as well.