I like the oddly-named engie mod now that the "HQ" factories have a visual effect. That is cool and I support it. I think it'll change FA for the better.
Not only do I think it'll be better, I know how: Right now, T1 engies are always better than factories at building units, with the exception of T1. This means that, in any situation where T1 is viable, T1 spam is the best strat. Anywhere else, T1 engie spam is the best strat. There's no argument, there's no debate, there's no "pros and cons", thus no balance and no strategy.
With the factory mod, factories will be made viable at all tech levels. T1 engie spam will confer better structure-building capability, reclaim ability, and the ability to quickly shift strategies (IE, moving T1 engies from a land fac to an air fac, or an air fact to a naval fac) relative to factories, which would require a greater investment of time and resources to switch. This is good balance and makes for good strategy.
However, the factory mod also buffs T2 and T3 engineers, which doesn't make sense. After all, with T2 and T3 engineers being as efficient as T1 engineers, why would you ever build T1 engineers after you had T2 available?
I propose going the opposite way and making T2 and T3 engineers more expensive, with less efficiency. Hypothetically, a T2 engineer could cost 300 mass and a T3 engineer could cost 800, while having 10 and 15/20 build power (compared to T1 engie with 5).
Now, you might ask, why on earth would this be good? Because it would create balance: T2 and T3 engineers have greater build opions and health than T1 engineers, and so it makes sense that they build more slowly. Indeed, the present situation of 1 T3 engineer being followed by 10 T1 engineers to make T3 structures shows a fairly good balance: T1 units are still useful late-game.
However, increasing the cost of T2 and T3 engineers will also serve to segment the tech tree more strongly between units and structures. Some view the tech three as very simple with one branch: Tech 1 --> tech 2 --> tech 3. We all know this isn't true, mainly due to difference factories. It's more like:
Tech 1 land --> tech 2 land --> tech 3 land
Tech 1 air --> tech 2 air --> tech 3 air
Tech 1 sea --> tech 2 sea --> tech 3 sea
I only wish to more clearly differentiate the last branch:
Tech 1 structure --> tech 2 structure --> tech 3 structure
If you have tech 2 land, that's great, but it should require a significant investment to also gain tech 2 structure. The 300 price point is fair because that's how much it would cost, with the factory mod, to double your T2 land output. That creates balance, and thus strategy: Do I wish to double my production power of T2 land, or do I wish to open a new branch of the tech tree, T2 structure?
It's all about balance.