I agree but with a bit of difference. I think instead of just being allowed to gain more after refusal of peace offer, you should re-declare/update your war declaration.
Let's look at it from an UI perspective
Name :: Type :: Status :: Length :: Options
War for 3 regions :: Conquest :: Won(Peace offer refused) :: 110 days :: <Declare Victory> <Surrender>
War for 3 more regions :: Conquest :: On-going :: 5 days ::
Declare Victory will send a message to the opposing party. If they accept, the one who initiated the declaration wins. Surrender will do the same.
Maybe down the road, we can have different types like pillage(ends when x amount of gold/resources were looted?), banishment(will end the war when the noble selected gets banished) and humiliate(upon surrendering, ruler will lose a big chunk of honour/prestige - could extend it to the entire realm but with different ratio of honour/prestige - ex: ruler 25%, dukes 20%, lords: 15%, knights: 10%)