The only reason why Krow is very powerful early on is the lack of additional requirements that have to be met. Most expensive units need assistance, either in the form of units (scouts, aa, riots) or in the form of buildings (radars, energy sources, grids). Need for unit support would make Krow unable to fulfill its expensive-high-mobility role, so that's a no go.
Now, a better way of making units less rushable and more mid-late-game than increasing their one-time cost is to add a steady cost, similar to that of cloaking units and Anni/DDM's grid requirement.
Here's some Neonstorming:
* A stockpiling dgun with a maximum stock of 1.
* Decreased HP:cost ratio so repairing drains more energy - energy is more expensive in early games.
* Energy cost to fly. Very very experimental.
I think cloning other games/tv and adding downward lasers would be boring.