I like it.
Sounds very exotic to me. Didn't even think to consider it for British royalty.
Years ago when I was a teenager, one of my mother's friends spent a rainy Saturday afternoon with me. She arrived at the house with a special edition of the 1970's 'Flambards'. I was entranced!
The heroine names her little girl Isabella (or Isabel). It's why I named one of my daughters Isabella. I have loved the name and all the many variants. It would be a wonderful name: Princess Isabella. Love it!
Likely as it is, well not exotic as much as foreign, French
There were four queen consorts of England named Isabella. Three were French. Isabella of Angeloume (2nd wife of John) Isabella of France (wife of Edward II) and Isabella of Valois (wife of Richard II). The forth was Isabelle of Gloucester who was English born but her family was from Normandy (1st wife of John).
In the main family line (kids of a king), there were two born. One was the daughter of King John. The other was the daughter of Edward III. John named his daughter for his wife (the 2nd Isabella). Edward III named his daughter for his French born mother.
I wouldn't rule out a name used by the Kents or Gloucesters, already used ones by the Spencers. I'd only rule out their own cousins. If we rule out any children (so remotely same generation) we'd have to knock out a lot of top picks:
-Edward (eldest son of the earl of st Andrews)
-Amelia (youngest daughter of the earl)
* note the third child is Marina Charlotte, so already used a Kent baby name
-Albert- son of Lord Nicholas
-Louis-son of Lord Nicholas and used for George
-Alexander-form used for the son of the earl of ulster
-Isabella- daughter of Lord Freddie
But I don't see Isabella. Not because of Isabella Windsor, but because I don't think they will go so afar in the family tree. After George and Charlotte I am expecting something far closer and more predictable.