Personally, I think around 40 players is a good & fair number to really feel like a 'guild', guilds having a low amount of slots will allow us to have many different guilds & it'll also help balancing any future GvG events.
But, having few extra slots is still not a bad idea tho I prefer making it a harder to get, like what @[ref]Synapse[/ref] suggested lvl based system sounds good or making something where all guild members will contribute on doing it (like what @[ref]Miles[/ref] suggested) .