I think if I'm understanding what John wrote, he was going to substitute "taxon" for "species (or lower taxonomic rank if it exists)" with the understanding that Individual is not intended to be used for aggregates of different taxa. That would solve this problem, right?
It depends on what you mean by "different taxa". If you are using the word "taxa" here to imply "species or lower ranks", than I don't think it would solve the problem. But if you mean it in a generic way, then I'm OK with that. By "in a generic way", suppose I had a trawl sample or a plankton tow sample that included unidentified organisms from multiple phyla, all of which are animals. I should not be prevented from representing this aggregate as an "Individual", with an identification instances linked to a taxon concept labelled as "Animalia". This means the contents of the Individual all belong to a single taxon (Animalia), and therefore it does not violate the condition excluding aggregates of different taxa. An instance of Individual so identified would be almost useless for many purposes, I agree -- but it's easy enough to filter such Individuals out by looking at dwc:taxonRank of the Taxon to which the Individual was identified. Also, it's not useless for all purposes, because a botanist would like to know that s/he doesn't have to look through that sample to find stuff of interest.
I guess my point is, there should not be any rank-based requirement for the implied taxon circumscription of an "Individual".
Rich