Proposal for experimental factory:
It would be super cheap (like 500 mass) and take up a lot of space. It would contribute only as much build power as a T1 engineer.
It's a T3 structure so of course it can only be built by a T3 engineer
There are 4 different versions of it, one per faction, BUT every experimental factory is able to queue up experimentals from all 4 factions
HOWEVER, you can only start making an experimental if there is a T3 engineer of the correct faction within the vicinity. So if you use a cybran T3 engineer to make Cybran's "experimental factory," you could start building an Ahwassa out of it, but ONLY if there's a Seraphim T3 engineer within a certain close range to the factory. Every time it wants to start making a new T4 unit, it performs a check of all nearby ALLIED units, if the check comes up empty, it skips making that unit and moves to the next item in the queue (the same way a T2 support factory will "skip" making a T2 mobile missile launcher if you have no HQ)
When a regular factory starts making a unit, that unit cannot be selected until it is done. But when this experimental factory starts making a unit, it CAN be selected immediately (like any other T4 that is being built)
Whatever waypoint (or waypoints/patrol order, etc.) of the experimental factory at the time the unit STARTS making would be given to that experimental unit. After the unit starts construction, you could change the unit's initial orders just by selecting it.
Any engineers that are ordered to assist the experimental factory would assist the T4 that is under construction until it is finished.
The experimental factory itself would not have a lot of HP. BUT if the experimental factory is destroyed, that should NOT in any way affect the experimental unit that is under construction (other than: losing the build power equivalent of a single t1 engineer and maybe the engineers that were assisting the experimental factory would lose their orders and stand there idle until the player gives them new orders). The player would still be able to continue assisting that experimental unit's construction.
It would also mean all you have to do is bring a friendly ACU with T3 engineering suite or a friendly T3 engineer next to an ally's experimental factory to allow construction of that faction's experimental units without actually gifting the engineer
This would allow players to reserve space to churn out air or land experimentals, without reducing the other costs (build power, mass, energy). It would only save APM without giving other advantages. There would be no adjacency bonuses.