Pretty much what it says on the tin, the ability to connect landing gear to wings/fins and have them deploy/retract laterally like on many WW2 Era Fighters. With the newly added curved wings it feels like not being able to attach gear to them and have said gear deploy laterally is one of the last big limitations in game with designing planes.
Having attachment on wings would be good. The workaround I suppose is to attach them to the fuselage (or somewhere else) and move them into position. Or place a thin pole in the middle of your wing and attach them to that.