I think the real answer to both of those is diversified defense, with a side of automated offense. This is not intended to be a personal combat sort of game, as far as I know, so any action-shooter conventions would be out of place. Factorio's combat is primarily in the tower defense genre, and right now you have painfully few options to choose from, all boiling down to direct upgrades in damage and usefulness. I'd much prefer turrets to be a varied bunch, all having strengths and weaknesses. For instance, taking the available in-game weapons into account:
- Gun Turret: Basic, small-profile weapon equivalent to your starting pistol - requiring said pistol in its crafting recipe, available early on.
- Defense Turret: Equivalent to current 2x2 gun turret, requires an SMG in its crafting recipe.
- Laser Turret: Better range than the Defense Turret and takes no ammo, but uses lots of power and fires slower, giving faster biters time to get close.
- Flamethrower Turret (yes I know it's being made): Will easily kill biters, but is outranged by spitters.
- Cannon Turret: AP or explosive ammo, massive damage, can kill biters and spitters both before they get into range, but slow-firing.
- Rocket Turret: Can easily blow up entire swarms by the time they approach, but slow-firing and can't target things that get close.
- Tesla Coil: Basically an electric Flamethrower, suffers same range issues and uses no ammo and lots and lots of power to fry anything that gets close.
- Gauss Cannon Turret: Ultimate long-range defense, uses both power and ammo, fires guided AP shots with massive penetrating power and great range, but relatively slowly.
If you have such diversity, you will be able to gradually change the layout of your defenses to better handle the changing monsters that attack you, rather than relying on damage buffs.
And of course being able to automatically stage attacks on alien nests using drone cars and tanks would be very very useful as well.