Just some thought:

The issue with size bracket is that is easier to get 15 active and well gear people than 25 or 30.

And also the 'casual' guild with 25+ people will probably most meet 'hardcore guild' pushing them to split to have lower level guild as opponent. 

In addition, the reward should then probably not be the same according to bracket

Currently we face stronger and stronger opponent, so i think they progressively fit people according to their strength

There should be a mechanic that match the average player rank between 2 guilds as well. My guild has about 8 players with rank below 40, while the other guild has the lowest player rank of 49 @_@. And we barely have 20 members, while duking against a whooping 26 >:( 

This situation could be easily overcome by calculating the shortage and either adding in AI units or matching smaller guilds.  Though part of the whole guild thing is you socialize with peeps that play like you.  Some people want to be special and have their own guilds...but if you can’t grow your guild, you should find one to join.  

Make it so that you get matched with guilds with around the same members as your guild

10-15

16-20

21-25

25-30

I agree. My guild has the same issue with only 12 members. =(

And it will probably get worse, I have to remove people from my friend list and guild at least once a week.
I just see that people stop logging for 7d+.

So I agree, wish the minimum required to get in the GvG was smaller