Every time I pop a drink-can I think of this one from my "Oh that's too obvious file"... but yet another search revealed nothing like it in the markets
It's a small cup that friction- fits the standard drink-can and does several obvious things.
 it seals in the fizz for a later swig
it serves as a cup for more elegant swallowing and more hygienic sharing to please health authorities etc. [A big advantage?]
[3 it could be made cheaply enough to give away world-wide with promoters' ads
 it could be easily made biodegradable or recyclable.
 it has a good protectable name to catch on and become universal ... KanKup.
 its introduction doesn't require redesign of existing machinery for making cans.
There must be a bug in the idea