For the HS 125
hydraulic flight controls would be nice
. But seriously: on the newer Hawkers (anything between 800 and 1000)
bleed air wing anti ice instead of this annoying TKS stuff would be worth a thought, as well as automated cabin pressurization.
Concerning glass cockpit retrofits: I am not under the impression that owners are willing to pay this kind of money right now since the only ones who benefit from this are the flight crews - and which owner cares about flight crews. But maybe they will be in 3 or 4 years when virtually every new or like new aircraft is fitted with a glass cockpit.