This is all really easy if you just keep the dogma out of the way.
All the medics are happy with the way the system is. So - A Faculty of Medicine Association (FMA) could retain all the functions of the ICSMSU. Easy.
Ditto for the Faculty of Engineering Association (FEA) and the C&GCU.
Then: as RCSU are happy to disband (an unbelievable piece of good luck), their clubs can move into the CSCs, as the RCS football etc, and slowly die out (as they have been doing for years). Education and Welfare Issues could then be established at a Faculty of Life Sciences Association (FLSA) level (ditto for Faculty of Physical Sciences Association (FPSA)).
RSM is the only issue, but there is no reason why the Union can't recognise tradition, and have, as part of the Faculty of Engineering Association a Royal School of Mines Union, administered by the FEA, so that people doing the traditional mining courses have somewhere to go... if that dies out in a couple of years, so be it.
And the issues are???