Tower Options
Right now, end-game always consists of either Missile Silos or Ion Cannons. Silos for long maps, Ions for shorter ones.
I think if we had more Towers and/or more upgrades for existing Towers, that gameplay would be less predictable.
Take for example, Bloons Tower Defense. Each Tower has two different upgrade options per level, and you can only choose 1 of the 2 upgrade possibilities. It makes the game have much more strategy, because if you upgrade a Tower along one upgrade path, the second upgrade path is closed. So for example, you could either upgrade a Tower's range, or a Tower's Attack Power, but not both for the same Tower.
Or create more Towers that specifically only attack certain Creeps, such as a Tower that only damages Slow-resistance creeps, rather than attacking all creeps.
Or maybe include more defensive Towers that affect other Towers already in play. For instance, a new Tower that does not attack, but when it's placed next to a Missile Silo, it increases the Silo's Attack Power or reduces its Cooldown.
Perhaps even including more Tower strategies for targetting the "Slowest" creep, targetting creeps that regenerate HP, or targetting creeps immune to Slows, would also help add strategy.
I think what I'm just saying overall is that it's good to have more options available, so that every game doesn't always start with Gun Turrets and Cannon Towers, and end with Ions and Silos.
Not sure if everyone agrees with me. Simplicity is golden, to an extent. But at the same time, after a while, you tend to expect what Towers everyone is going to build. And with only 6 Towers available, there are few options, especially during end-game, when you have to use Silos and Ions, no question about it.