Let me just preface by thanking the modding community for all the fun I've had these last 6 years playing KSP.
One thing I've wanted since the time of Lazor systems and 50/50 odds of instaKraken on symmetrical craft is one (or few) - piece wing roots. These would contain fuel (less shocking in modern fuelled wing days) and have attachment points for engines or maybe even small payload bays.
By matching the part(s) to stock wings it would help both with wing extensions for larger craft and would be more efficient in stock and FAR than the draggy/heavy DIY alternative. Given the recent pivot to early/pioneering designs in space, why not parts that reflect the state of the art in aircraft during the same period? On top of that, with environmental concerns putting blended wings back in vogue, I'm sure with seamless transitions between fuselage and wings some pretty futuristic craft could be created.
Engine mount wing root
Inline payload wing root
FAT - 455?
Comet (1st ever commercial jet)
HP.80 Victor (tanker variant of strategic bomber)
FAT - 455?
Halifax (WWII bomber)
Big - S Delta?
Avro Vulcan (nuclear bomber)
Modular wing size?
B-2 Spirit (modern stealth aircraft)
Modular Wing size (Inlet only)?
F9F-8 Cougar (early jet fighter)
Sea Vixen (1st gen supersonic carrier fighter)
Just for fun, some decidedly less historical blended/loaded wing designs