This is going to sound snobbish, but the sort of people who buy a seven figure house are probably going to be upper-middle or middle-middle class and will be turned off by a lot of home 'improvements'.
Restore or re-instate period features, such as cornices and chimneypieces.
Overhaul, but don't replace, original windows. If they can't be repaired, exact replicas of the original windows in hardwood double glazing only, never uPVC.
Good joinery in a style suited to the age of the property - solid (ideally original) wood doors and generous skirting boards.
Wooden floors. Ideally the originals if you can restore them; if not, top-quality engineered wood.
An Aga or a Rangemaster. If you want built-in appliances, and will be leaving them behind, Miele appliances.
Don't overspend on your kitchen. Everyone has different tastes. But you can't go wrong with a painted Shaker or Spitalfields (Shaker with an Ovolo bead detail) style kitchen for this demographic.
Bathrooms are again very personal but if in doubt, go totally classic. Lefroy Brooks Classic range will never date.