So far, Bisq has a modest size, is another reason why it would not be a danger to Bitshares, and has just what Bitshares needs: a Fiat Gateway full p2p. I think the time has come to add and not to subtract. It is the way in which all the big companies have grown.
I do not know if you realize another advantage of using an external p2p Gateway to Bitshares: If a node incorporated some FIAT exchange system, even P2P, there could be legal and regulatory problems for witnesses, depending on their country.
However, since Bisq is an external App used by each user, those problems do not arise.
I think this has the potential to be a great idea that should be explored.