We have a strategy called The Perfect Defense, so why not "On The Offensive" ?
You could have all the offensive units higher in attack, but all the defensive units week in defense.
So infantry and milita could have -2 in defense, and Marines and Tanks, bombers etc... will have +2 in attack.
Also Transport (Air and sea) could be more expensive.
Your idea's on this?
If you don't like it, how would you improve it?