Combine Mortar.
If you remember the part in the fight for City 17, where you reached a huge museum-looking building, on top of it's roof there was a Combine Mortar that you had turn off via shotting off energy balls from the thing that held them.
https://www.youtube.com/watch?v=cZotpNbQNkY
^This.
I am learning particle editor now and it seems quite fun and not hard to learn. I already managed to make a few particles that look like this mortar effect

As for the ballance: Since the Rebel explosive barrel tower uses some scrap to shoot those barrels, perhaps combine mortar would use only Energy (but in huge amounts) to shoot the charges? The thing should also deal at least 100 damage to all units and have a bit increased cooldown. It should also NOT have a big enough range to reach another base.
Next,
As you probably know, Strider is a flying entity (which means it does not walk), I was thinking, maybe you could make npc_helicopter, npc_gunship and npc_dropship flying entities as well (so they work without path_corners)? If those are included then we probably need something what can counter those. Lets think: rebels managed to create a jeep which has this laser attachment. And it seems they made it at quite a small factory there (which was surrounded by combines). Another thing we saw is an airboat with the helicopter machine gun attachment. I know these were already suggested but my idea is perhaps you could make an upgrade building (just like Armory) but for vehicles?