Western Mountaineering or Feathered Friends are the only options IMO. Both make semi-rectangular models. I'm the same way, cant sleep in mummy style bags, absolutely love my Feathered Friends Penguin 30. I'm a hot sleeper and I was comfortable in temps into the mid/lower 30s. I'm planning on ordering a Western Mountaineering bag for late season, also in a semi rectangular. Can't beat these companies, essentially a custom, handmade in the USA.
Big Agnes makes great stuff but when you start comparing specs like fill weight, there is now way a bag like the storm king can keep you as warm as a bag like the WM sequoia.
Storm King 0 degree - 22 oz of 650 power downtek
Sequoia 5 degree - 33 oz of 850 power with microfiber shell
I'd like to know how the storm king can say its a warmer bag than the sequoia, because going by those numbers, it is not thermally possible.
Look into the WM Ponderosa MF 15 or the Feathered Friends Penguin Nano 10
I can promise you will be a happy and warm camper for a very long time.