For the A321XLR, Airbus plans to eke out more space for fuel by moulding one tank directly into the fuselage, meaning its shape would follow the contours of the jet and carry more fuel.
Not the wing but the fuselage .
Some years ago I few an innovative aerobatic aircraft where the fuel tank extended in the space between my legs. This was supposed to be a brilliant engineer idea to keep the aircraft as aerodynamic as possible and the most weight close to the CG . A brilliant idea indeed on paper but having only a few millimeters between your most valuable parts and a plastic fuel tank made sure I did not renew the experience. The aircraft never went beyond experimental.
Sometimes what looks like good ideas fail to succeed in practice. I hope this one does too. ..