Define "new" :P. If by new units you mean entirely new assets then shoo, you. Else, yes, I can add new units by copying other units or using the assets already baked. Like the Scarab. I copied the essential properties from the standard one to then create a "UNSC Scarab". IE, and Scarab that starts as Cutter's personal scout unit and takes incoming fire like a windshield does rain :P. Oh and level 3 bases can train new ones for 5 units and $2K. Arbitrary numbers of course.

Also modified the Barracks to support building upgrades, much like supply depots and reactors. I added 2 upgrades to the Barracks, with each upgrade changing the train unit selection. A level 2 Barracks could train Spartan Omega team units and a level 3 could train flood units.

At any rate, there needs to be a Halo Wars 2 before there's a Halo 5: The Chief Strikes Buck. Else I'll stop giving kittens cheeseburgers.

Think of the kittens 343...