The humble layer mask mechanism is core to so many implementation techniques in Unity that for literally every project I work on it always seems that 32 layers is simply not enough.
In the past I've worked with various proprietary game engines where we would start off with a 32 bit masking system and by the end of the project have always increased it to 64 bits! Not being able to do this in Unity forces so many hacks and layer re-use tricks that I can't believe this isn't a major source of bugs and irritation for the whole community!
My proposal would be an option to use either 32 bit or 64 bit layer masks.
Even better (or an acceptable option if the above isn't practical) would be another option allowing you to specify 'use separate render/physics layer masks'. ...Having two such core systems fighting for the use of just 32 bits forces us to make compromises in layer naming and/or to have to reserve blocks of layers just because some third party assets require them.