Kind of an out-there suggestion that I doubt will be seriously considered, but it could be used to stretch out the relevance of ground experimentals.
Rather than making the experimentals simply better through increased DPS or health, the experimentals are made into much more versatile units.
Here's some ideas for what this could look like:
Slot 1: Air factory | Improved engines
Slot 2: Advanced AA (aircraft carrier grade) | Tactical Missile Warfare System (equivalent to four TMLs and TMDs, builds TMs with reduced price and damage and scatters when launching a volley)
Slot 1: Disruptive Beam (beam has increased shield damage) | EMP Overcharge Cannon (manual/auto fire like ACU overcharge, requires a huge amount of energy to recharge)
Slot 2: Rapid Reclaim System | Rapid Capture System (tractor claws reclaim/capture units at a very high build rate)
Slot 1: Incendiary AA Artillery | Portable Gateway (moving units to Ythotha teleports them there at the cost of energy, with enough energy you can even move experimentals)
Slot 2: Amplified Detonation | Tactical Nuke (Like Billy)
Slot 1: Stealth field -> Cloak Field (no self-cloak) | Stationary Fire (improved range when not moving)
Slot 2: Bubble shield | Radar Jamming
Slot 1: Flak Battery | Long Range Missile Deflection
Slot 2: Artillery Guns (similar to three gunthers but with 100 range) | Advanced Engineering Station (equivalent to two maxed out hives with double range)