First, the goals: Games should be time limited (you can pick the time amount, but for QB lets assume 20 minutes) and there should be no incentive to make them as long as possible; draw kiting should not be a thing, corner camping against white lines should not be a thing, and players should not have disagreements about where they are "supposed" to fight (e.g. in a forest or not) that result in standoffs or arguments about whether to fight at all.
Aside from those goals the game's tactics should be not affected; wide open and sandbox.
1) There are two possible game ending win conditions - destroy the enemy army, or accrue XX cumulative minutes (assume 20 min for QB but customizable for custom battles) of flag presence before your opponent does.
2) There is a flag and small box in the center of the field, too small to evade enemies inside. In almost all maps this will be on open ground (and it can't be on impassable ground).
3) Any player with any ground unit in the box at any time for more than 5 seconds starts accruing time on their flag presence clock. Both players can accrue time on their clock at the same time if they both have units there.
What does this do? It solves any argument about how to win if neither player is otherwise motivated to engage the other, while being very unlikely to be triggered in the vast majority of games before the enemy army is destroyed.
Q: But what about all treekin or zombie armies designed specifically to exploit this to last 20 minutes and be unkillable in a normal game length?
A: Even if such armies exist and are lucky enough to go against an army that can't kill them in 20 minutes, such armies are still slow. In this worst case scenario, you could simply move to the flag first when you see the build they brought, and they would likely be unable to fully dislodge you with their slow grinding defensive army. At a minimum you'd have head start.
Q: But what if I want to play a kiting game and I might need more than 20 minutes to win the way I like to play?
A: Whatever time limit you set on your games, or whatever CA sets for QB, that's the intended max time limit for the match. You should take armies that can accomplish the goal of winning the game within the game's maximum time frame. Using ammo supra optimally for longer than the maximum time for the game you've chosen to play simply isn't polite to your opponent. Agree on a longer game with someone if you want the option to play longer games.
Q: But what if I take a defensive box with artillery inside and then they snipe my artillery and i have to advance on the enemy or the center to win?
A: Yup. You lost the ability to win simply by standing there and shooting after they killed your ranged units. So now it's clear what you will need to do if you want to win (engage your enemy or contest the center).
Q: Won't this encourage certain army types? Aren't melee heavy armies disadvantaged? Aren't ranged armies disadvantaged?
A: The only army type that is discouraged is one that takes more than the maximum allotted game time to kill the enemy army, some of which would be out in the middle of the open field the entire time. The only army type that is encouraged is a non-existent one that is somehow fast enough to claim the center first and also tough enough to reliably last longer than the maximum game time.
Q: Will this make games take longer or play around the center of the map only?
A: Nope. in fact wins by timer runout will be exceedingly rare and it will rarely be of any real value to pay attention to the center of the map at all. it will largely just avoid disagreements about "where to fight". Any units standing in an open center will die faster than they would otherwise anywhere else on the map.
Q: What about good games that legitimately last longer than XX minutes?
A: This happens sometimes and currently results in draws in QB. Under this new system, games will still often last longer than 20 minutes, simply because most players won't be focusing on the center the whole time, if at all. if both players are actively engaged in fighting elsewhere, they can fight for as long as they like. But if one player wants to start the clock to avoid an excessively long game, they have the option to return to the center with a unit.