Easier rebuilding would make a big difference.
Keep track ingame of each nations max infra level (the highest they have had for each city) and when rebuilding they can re-buy up to that level at a reduced cost, say 30-40% of the norm. The justification could be that they are repairing damaged but existing infrastruction instead of building it from scratch.
You might also consider 'Warmonger' monthly rewards for infra destroyed, soldeir kills, tank kills, etc. Have a scoring system relavtive the the nations score so that all can compete. There could also be alliance equivelents of these based on all allaince members contributions and relative to the alliance score, but make the alliance reward a treasure that can't be stolen and moves each month to the new winner of that category.