This commonly used operator forces the use of advanced queries, thus lowering the "build to change". But, on the other hand, simple queries should remain "simple"...
I propose keeping simplicity of simple queries by having the "IN" SQL operator available referencing a previous query output field (thus, "IN" the values of field X returned by that query.
Example of such an expression (for a simple query condition):
SIMPLE_QUERY_ENTITY.EntityB_ID IN PreviousQuery.ENTITY_B.ID
Of course, ideally, the usual ".List.Current." part of the record list expression should be omitted since they would affect readability.