I don't know about a designated marquee (think it's a fair bet though that whoever is signed as the international marquee will probably be one of the biggest international names to ever play in the A-League), but the amount of transfers from City to Heart on loan deals will probably hinge on what rules the FFA decides to implement, based on this unique partnership. If they do nothing, it's pretty obvious to see how easily we could abuse the standard loan deals, so it's likely that if Heart does get City players on loan, the rule would either be that Heart needs to bear some of the cost in the salary cap, or that there is a limit allowed (probably one) per season.
An Australian marquee would be nice as well.