Bit of googling suggests this is a whole, fascinating world of little improvements. However it's also both constrained and pushed forward by what's road legal.
https://www.volvotrucks.co.uk/en-gb/news/insights/articles/2... : removing mirrors allows for much smoother cab airflow
https://go2stream.com/blog/aerodynamic-truck-legislation-rea... : UK legalization of fishtail-like devices
https://www.kudauk.ltd.uk/aerodynamics-explained : Kuda on the UK allowing higher loads, and therefore benefiting from extra wedge devices on the top of the cab.
I'm sure there's a lot more out there. The eventual switch to electric will probably come with another round of aerodynamic improvements to maximise range, as with cars.
> Kuda on the UK allowing higher loads, and therefore benefiting from extra wedge devices on the top of the cab.
Cab-top deflectors are extremely common on every truck where the cab is not tall enough to cover a standard trailer (which is common, usually only the highest end sleepers are that tall e.g. Scania's highline cab on the R and S, Volvo's globetrotter xl and xxl, ...)
For instance on this hero image from scania's site every truck but the very shortest and the very tallest have a deflector: https://www.scania.com//group/en/home/products-and-services/...
Obviously if you run higher than standard trailers, you need a custom deflector.
> removing mirrors allows for much smoother cab airflow
On brand new trucks (designed without mirrors) yes. However on older trucks the mirror design might be an essential part of the aerodynamic design and required for a low drag coefficient.