Here's a list of the specialties that I believe should be available to expand, some replace ones we already have.
Internal Medicine - General Surgery
Paediatrics - Paediatric Surgery
Cardiology - Cardiothoracic Surgery
Neurology - Neuro Surgery
Orthopaedics - Orthopaedic Surgery
Burns & Plastics - Plastic Surgery
Obstetrics & Gynaecology - Ob/Gyn Surgery
Intensive Care
Critical Care
Emergency Department - Trauma Surgery
I would however recommend with this that each expansion comes with its surgical alternative or there's one expensive expansion for all surgery as irl surgical use the normal wards and stuff from there medical side of things and then just have theatres/operating rooms.
Also it'd be cool if there was an oppurtunity to have a hospital specifically for one thing for example paediatric hospital, general hospital, maternity hospital etc. Therefor all departments in that hospital would be specified to that hospital for example in a paediatric hospital or the departments are paediatric however in a general hospital they have a paediatric unit which is only for general medicine and specialist paediatric patients require transfer to the specialist hospital.