It would be nice to have the ability to upgrade a clinic to a hospital ,in the same way we can upgrade a small fire station to a large one
Clinics were added for the US game for a bit of realism in terms of real life medical buildings in the US, however you have to look at the idea that when you start the clinic you get the space for 2 ambulances and 10 patients plus it counts as an ambulance station, when you upgrade where would these perks go? Because is you have 2 ambulances thats 10k and if you bought staff with coins thats 20 coins. So im my view while it would be nice to be able to do this, could you compromise and give the clinic more beds as in reality its long term goal is not to become a major hospital in the region rather its purpose it to provide local high quality (hopefully) medical services in the region that it is situated in.
In my setup I use the clinics for A&E (ERs for US players) and then the main hospital in on the same site to give a feel of realism as typically unless pre arranged most patients brought in by ambulance get transferred to A&E for assessment and then sent off to the department ward that they need.
Hope this helps.