Why should the Habsburg loose even more powers.
In a Federalized A-H their role could still be akin to the President of America i think and with ech state having his own Governor.
Another Ausgleich would just cause ven more trouble, because then we have unequality again, the best thing would be the same equality for everyone.
Big problem here is that you envision a USGA as a version of the actual USA, but the Croatian, Hungarian etc. etc. well IMHO think more as a CSA like nation or as the immediatly post ARW USA and in this kind of scenario the role of the Hasburg will be very very diminished (an eresy for them)
Also a USGA scenario will nly be possible in a CP Victory world, meaning that Serbia and maybe Romania ( depends if they go on the Entente side or not, as Romania entered the war pretty late, so i rather think they stay neutral in a world where the CP have an advantage over the Entente) could get annexed as they were war goals of AH
First absorb Romania and/or Serbia is out of question too much problem, as in OTL A-H only get economic concession and limited territorial annexation and of strategic value from Romania.
Second if this is a CP victory scenario, well unless the war is short (basically only a conflict with Serbia) and victorious the empire is in deep deep trouble, think of Italy after the first world war in term of economic and social trouble (the biennio rosso), but with more deaths, more damage and the ethnic problem so yes maybe emperor Karl can try a move like this to salvage what he can but he is really lucky if it last a year tops