It would then come down to what sounds suitably grand and historic, and isn’t already taken by some non-royal inheritance. Not a great deal of room to manoeuvre. “Duke of Milton Keynes” somehow doesn’t work.
Hohoho.
There was Northumbria, and very influential it was - but then the Danes came.
https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1mXWo6uco/wiki/Kingdom_of_Northumbria.html
After the Union with Scotland, they became quite creative, since they wanted to give royalty titles with links to both England and Scotland, and this involved utilising placenames not previously linked with royal Dukes. So you have Dukes of “Cumberland and Strathearn”, “Clarence and St. Andrews”, etc. And in time Irish place-names come into the mix as well - “Connaught and Strathearn”.
In more recent times, however, the fashion has been for one territorial designation attached to the ducal title, with subsidiary titles referring to places in other countries of the UK. So the Duke of Edinburgh, for example, is also Earl of Merioneth (in Wales) and Baron Greenwich (in England). And this has meant a return to more traditional place-names associated with the ducal title.
Alex Salmond got into a big row for calling William and Kate the Earl and Countess of Strathearn.
Dear me! Then think of the row you’d get into for calling them just plain William and Kate!
So the Duke and Duchess of Sussex will live in Suffolk… Nice wedding prezzy though 
If it’s true of course?
It ain’t ;). It’s not fit to house anyone, it’s not a royal property and is managed as a public tourist attraction. That paper also suggests that Donald Trump dressed as the Queen to crash the wedding, which seems…unlikely. Maybe :D.
A news source with the strapline “You couldn’t make it up!” 