Workers, and get rid of that extremely stupid idea diminishing returns. The affect of getting an expansion should not take 5 minutes before they start to go plus. Could also make it so the supply pad can take less damage or the more damaged they are, the less resources they give. With workers or fragile supply pads, you can harass the enemy's macro, right now you can't do anything game changing to their econ like a perfect "storm drop" in StarCraft.
Free base building of course, LoTR BFME2 and CC3 had it and it worked really well, why shouldn't we be able to do it? Free base building adds so extremely many more strategies it's almost scary. I'd like a third faction aswell.
Two resources. One normal and easy accessible, and one not as accessible but required to get the better units and upgrades. A scorpion could for instance cost 425 Silver and 75 Gold, and gold is only available at your expansion, so you had to expand to get the better units. If there was free base building, so you could put towers up imidiately, and no diminishing returns, it would be worth it.
Late game UNITS instead of Late game upgrades. There are no late game units in HW, only upgrades. You just upgrade and upgrade your tanks untill you win or loose. There are no Ultralisks in Halo Wars, and I think that late-game units like Ultras should be added. But they shouldn't have the ridicilous price of 3K like a Scarab. You should be able to pump them and use them, if you're able to get into end-game.
"Spellcasters". Some units with powerfull abbilities. Like a weak sniper that can use a lazer designator to target an area where a mini-mac will blast, and other spells, some defense-spells aswell.
And force people to expand, no infinite resources out of one base!