Full Idea
We could introduce an 'intentional quantifier' (Ix) which means 'some of the things we talk about..'; we could then say 'some of the things we talk about are F and exist' (Ix, x is F and x exists).
Gist of Idea
We need an Intentional Quantifier ("some of the things we talk about.."), so existence goes into the proposition
Source
Colin McGinn (Logical Properties [2000], Ch.2)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.33
A Reaction
This immediately strikes me as a promising contribution to the analytical toolkit. McGinn is supporting his view that existence is a predicate, and so belongs inside the proposition, not outside.