Unit Upgrades: Self-explanatory. The game needs this because customisation of one's army should be as in-depth as possible, adding more strategic possibilities and length to games. Plus, it gives more things to spend Etherium on.
Unit Abilities: Beyond just heroes, units should have special powers that they can use. Not all need one, but there should still be plenty of units who do have powers. They should all have cooldowns, as well as a low Etherium cost to use.
Edit: More suggestions that I didn't notice.
Unit suggestions are rather specific. I'll add them to the list, but keep them in a spoiler, and leave broader desires outside.
Here are mine, made to coexist with other previous suggestions from Kevin and Xstar, who seems to like making low-damage units.
Earth Piercer- A slow, heavy mecha with a very high cost, average life but high armor. It has no main guns, but an extremely powerful ability that can take some time to recharge. It launches 2 rockets, one per arm, that deal incredibly heavy damage, instantly killing most units and small structures in a large AOE. The explosions leave a wide hole in the ground that prevent the movement of infantry, but the holes disappear eventually.
Deathbringer- A Melee infantry unit with high speed, given by rocket shoes or a rocket pack. He wields dual blades made of concentrated Etherium that can slice through the thickest metal or energy shield. He deals high damage to units but is easily killed by ranged attacks.
Warden- A slow-moving mecha with dual turrets that can lock down in a location, gaining increased firing speed, damage, and armor. It takes significant time to alternate between each mode. Alternatively, it must stay locked down once it does so, but gains an even more powerful stat boost compared to the 1st concept.
Etherium Guardian- A flat platform, surrounded by metal shields on all sides, and moving on tank wheels. In the middle of the platform is a large spire, conducting power. Infantry units around it gain energy shields that recharge after not being attacked for awhile.
Sentry- A flying unit, in the form of a small, light mecha with an infinite jet pack. It has a slow attack compared to machine guns, but not that slow, perhaps a mere bullet per second. The bolts would be of pure blue energy, exploding for a small AOE and dealing bonus damage to enemy machinery, be it structures, mechas, or vehicles of some other kind. It is quick, powerful, expensive, and vulnerable to damage.
Arcane Omniscient- An expensive infantry unit that sees an alternative use for Etherium: to find a greater life, a greater world. They cast devastating spells that can deal massive damage to a large army or instantly wipe out a single, elite enemy unit. Expensive and frail, they are glass cannons.
Purifier- The ultimate mecha, it has average speed and good damage, with extremely high defense. One of the most costly units in the game, it is a statistical monstrosity, either for late game assaults on a heavily guarded base or rushed out to be micro-ed to victory as it slaughters weaker units on its own.