I've been thinking about the YF-23, the Su-57, and the J-20's weapons bay.
When you look at the YF-23, you can see clearly how the plane is designed. You have a pilot pod in the front, two tubes for the engines, the wings, and the pelican tail. The weapons bay is supposed to be part of the pilot pod. On the Su-57, they use a similar concept; except that they deliberately avoid having the engines placed adjacent, partially because the Russian 3D TVC concept benefits from having the engines at an angle. So there's a space between the engines where the Su-57 can jam weapons bays.
On the J-20, the fundamental design is that at the front, you have a pilot pod, with intakes to the side, canards on the sides of the intakes, and LERX forming behind the canards. You have a weapons bay placed between the pilot pod and the engines, which limits the size of the weapons bay.
But what if, say, with mature WS-15 TVC engines, the J-20 used an engine configuration similar to the Su-57? That's to say, let's start by going tailfin-less and relying on the TVC to do yaw control. Drag is reduced to begin with, but where the tailfins used to be, you plant the engines instead. Then you have a huge chunk of space opened up for a central weapons bay, as with the Su-57, but since the weapons bay is wider than on the Su-57, you have almost double the weapons bay room on the Su-57. Then of course, you extend the delta wings to the rear to shroud the engines, and place ECM modules at the exact rear of the J-20 instead of on the extending pylons.
It'd be a substantial change, but you'd have an aircraft whose weapons payload would compete with the JH-XX. Could be an interesting J-20-derived analogue to the Su-34 or just the base design for a J-20 analogue to the Su-30 / Su-35 line (i.e, strike and omni-role).
Edit: Realistically, looking at the J-20, you couldn't move the engines far away enough from the centerline to create enough space to double the J-20's weapons bay. But you could, however, manage to create a long tube extending from the current weapons bay center. Or, in other words, create enough space to launch 2 PL-21s with enough space left for PL-15s behind them.
