Pike, Long Spear. Problem Solved 
Useless comment. You can't expect people to change their class, otherwise it would be completely unreasonable to offer other equipment and skills to choose from on the website.
Anyway, regarding the op. I don't think cavalry is overpowered, and I don't think it's any problem with the banner balance and having the majority of one class on one team.
Nobody said cav was OP. But in a rock-paper-scissors-system, which is basing on the fact that all entities have the same chance of killing someone and being killed by someone, you have serious issues as soon as you have ten rocks fighting five scissors and five papers, because there won't be enough papers to stop the rocks in time, and then the rocks will smash everything. The same mechanic applies for archers, too. The only exception is infantry, as - in difference to the other classes - they both lack mobility to run away and flexibility in which target to attack. Archers and cavalry can support each other even without teamwork, infantry can't. A linearly growing percentage of infantry increases the effectivity of the team linearly, a linearly growing percentage of archers and/or cavalry increases it exponentially. There's the difference and the main problem.
You can still if you play well and try to organize the team. It won't work all the time, but once you get an urban map then the GK team will have a real problem getting multipliers at all. I don't see anyone complaining about that though, when they're able to steamroll the other team who consists mostly of cavalry who's obviously a bit weaker on the ground.
A full GK stack will probably win most of the time on very open maps like ruins and the open deserts etc. but there's also urban/city maps in the rotation, don't forget. They pay for their efficiency on open maps by dedicating themselves to being cavalry, but it also brings that bad part, they'll have difficulties when the maps are less favourable.
First of all, with the current emphasis the developers put into organizing tactics and leading teams (which is... err... zero?) you can't really expect the other team to always play with their mind and have equal chances. I tried it, and there was a time I expected it, but I gave up. Most cRPG players just play for their personal duels, not for winning the battle. I see tons of gameplay videos where melee fighters are only moving towards the first target they see, in a straight line, neither looking for cav or archers nor what the rest of the team is doing. It is pretty frustrating, and I am starting to be one of those who always claim that humanity consits of 90% idiots. At least. You don't see anything else than autowalking Rambo-lemmings, of which most should be allowed to use those parking spaces with the man in the wheelchair sign on them. That is why "organize your team" is not an acceptable answer for someone who DOES care for winning the round, which puts him in a minority in cRPG.
Then there is the other fact that we have very very few maps which are ONLY city maps, but a lot of rather open maps which are well suitable for cav. To say that it is fair and cav has to pay their prize, you would need a percentage of 50% city street maps. But you don't, so it is unfair most of the time. And by the way, cav does also not really bad on those maps. At least not as bad as they do well on open maps.
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And I find it quite symptomatic for the cRPG community how none of the GK players showed that he actually understands the concerns of some players, and that he can feel with them. It's only about being as effective as possible, and growing the own e-peen by collecting kills, instead of actually feeling like a community which holds together and is aware of the fact, that you need the other players to be able to have fun, because fighting bots is not really comparable. So people need to learn that caring for other people's fun means securing the fun for oneself. But see my paragraph above, and of what 90% of humanity consists of.
