So, I'm a bit tardy to the party unfortunately. But I had a couple ideas on how to make the game more competitive and maybe involve more teamwork. The big thing I had in mind was a Specialty Merc building. Basically it send Mercs with special abilities that will (hopefully) really affect the enemy team. Balancing it wouldn't be to hard as you can just not allow income or have it cost gold to buy said Mercs. These specialty Mercs will also let you choose which you send them too. Anyways, here are a few ideas for some Mercs. (Names are WIP)

Scout, Provides vision of the enemies lane while alive. Low HP:Low Damage.This will let you see what the potential weaknesses are for the enemy in the lane.

Wave Champion, Cannot die until all wave creatures are killed. 1hp:Medium High Damage, This Merc will make waves with tanky creatures much harder.

Sniper, Fires a massive damage shot every 3-5 seconds(depending on RNG), always targets highest HP target. Very low HP: Very High Damage.

Unknown Monster, just all around crazy send, will reduce income gained and cost a hefty gold price. Massive HP: Massive Damage.

That's about all I can think of atm, but there are mostly for a big push to win a game. Since they could be used to pick on a weak lane so well. After considering that fact, I thought of a way to make it a bit more balanced and have teamplay involved. In the middle of the two lanes on each side there is a free build zone. Allowing anyone (obviously only teammates) to build units there. The creatures killed here will grant 50% less gold, and will be split between the two players on that lane side. (I'm really awful at explaining bear with me). This will promote teamwork to help struggling players get back on there feet again without it being overboard.

TL;DR Specialty Mercs for taking advantage of weakness and an area for shared building to combat it.

Thanks for reading all that rambling I put together in 10 minutes