Great idea: Enemy's speed.

[ValeriusPaipikus]

Speed of enemy should not only determine time units, but also how hard it is to aim and shot it. Faster enemies should be harder to aim at, so minus to accuracy of shooter. It should be easier to aim and hit slow enemies, so + to accurasy of shooter. Units with overweight are slower, so... Got it?

Don't forget that battle is a simulation, and so must be realistic.

[ohartenstein23]

Doesn't work well in turn-based games with simulated ballistics. Force your unit to fire one tile short or beyond the target and your speed penalty suddenly doesn't factor in.

"Must be realistic" is not an approach I'd take here. We're dealing with a game about alien invasions; realism may be incorporated, but it still takes a back seat to the idea that the game should be fun. The feature as you propose it wouldn't add much fun, just me and a number of other players finding ways around what we think is an unnecessary penalty.

[LuckyClover]

units with lot of time units have allredy this advantage of showing from nowhere and atacking You on theire turn-
like chryssalids who reach You from far distance and You cant do nothing about it-it is turn game way of showing speed
Youre idea is cool but this game is hard enough for me even without it so ... 

[ValeriusPaipikus]

>Force your unit to fire one tile short or beyond the target and your speed penalty suddenly doesn't factor in.
But isn't there 2x penalty because units are blocking sight on space behind them? This is good argument, i wish it to go further.
>the game should be fun.
If it isn't fun to you than it only means that we have different definitions of fun.
>but this game is hard enough for me even without it so ... 
It is always possible to play on lower difficulty