I don't disagree with this assertion. I would like to be able to "hybridize" the individual units to create new ones (not necessarily entirely new features). For example, I was thinking it would be cool to have a tank, which would (effectively) be a cross between the artillery and infantry (unsurprisingly).
To clarify the specifics of a (potential) implementation:
Big green cube (not four tiny ones)
Slower than infantry (would need to be fine tuned)
Shoots slower (tunable)
More "accurate" (tunable)
Splash damage (like artillery, but not as devastating)
Low building damage (takes, for example, four hits from a "tank" to collapse a large building)
There are few balance issues, but I think that it would be pretty cool to combine basic mercenary types. Some other ideas:
Infantry or Snipers + Bait (Trappers?)
Infantry + Artillery (Mortars? Two Man Team, Less Destruction, Less (Area) Damage, Higher Fire Rate, More Mobility, Balanced with "Setup Time" of Mortar)
Infantry + Evac Chopper (Less Capacity and/or Less Speed, Defend the LZ)
Infantry + Barricades (Entrenched Troops, Non-Mobile, Limited Field of Fire (Sniper Style?), High Fire Rate)
Snipers + Artillery (Precision Ordnance? Against Big Zeds, for example. Like spotters in the military)
I understanding the implementation/balancing would be a potential issue, but it would still be very cool.