HT has addressed the preponderance of the top five popular planes (more commonly known as the "dweeb planes") indirectly, by adjusting the perk ratios. Although its early, I am seeing a few less of them and a bit more variety in the air -- not much change, but some.
Why not use the same indirect approach, tweaking rather than more radical change?
Now the most populous country gets fewer perks for kills, and pays less for perk planes. This helps play balance some for the night, and indirectly for the campaign.
Why not just add a multiplier? Double the ratio, so the outnumbered country gets twice its current perks for a kill, and finds perk planes half their current calculated cost. If that's too radical, then increase the ratio by 50-75%.
I really think this would work well, without adding a completely untested element.