Killing the darbars is just an annoyance that makes it harder to find a fight in a combat game.

My personal favorites for an alternate result of a destroyed HQ:
1.
Disable all AA. This means fields, towns and even CV groups. This would be a big incentive for the long and dangerous trip to the HQ and also much more incentive to defend it. HQ battles would have a huge significance, as they should.
2.
Disable base capture. Supplies would still work and troops could be deployed, but an arena setting would keep any bases from being taken. Most people like to be on offense. If their troops are going to be nullified, they will be more inclined to defend the HQ. Big fights at the HQ would happen, especially when a map is almost won.
Also, add my +1 to upping the HQ damage requirement so that a single set of Lancs can't take it down.