50 or more Diamond guilds, so the chances of 2 of those 5 guilds being in the same league are pretty slim,
It's better then 50% that 2 of the top 5 Guilds will share a League.
If the first four are spread out over four separate Leagues, the last one has only 3/7 chance to be in a League without one of the first four.
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As far as the rest of your post, sure, 5 Guilds dominate GBG now in your world. Things can and will change. Players get stronger, shift Guilds, alliances and enemies shift.
GBG is 14 months old.
A lot of players and Guilds took some time or are still figuring out that GBG is a treasure house and how to take advantage of it.
While the top players have huge leads and will maintain, even extend those leads, the percentage of that lead (thing second differential) is shrinking.
The Guilds who want to get better at GBG are looking for and training more good growing players and working on expanding their Guild Treasuries.
Those were slow processes in 2019; with GBG not so much anymore.
Assuming no major cahnges to GBG or new aspects added to the game of similar effect on the game as GBG, I'd expect a slow increase of the number of 'tier 1' Guilds on every world.
So while you may not always have partners to dance with now, give it time, you will.
Or, oh, I dunno. Work with some of the weaker Guilds to help them get better so that when you face your enemies over the coming months you have more friends and allies to count on?
Ridiculous? Yeah. While you may think grooming potential new dance partners is silly, you can bet others don't see it that way.