For my closest collections over in Spain (can't say I visit local collections in the UK unfortunately so am less able to comment):
Bioparc Valencia
Honestly would love to see a bunch of species for their respective areas, as there is lots of potential if an expansion of any sort would occur. Carnivores such such as Cheetahs, Caracals or Vontsira, Primates such as Aye-Aye or Gelada and Hoofstock like Black Rhinos, Cape Buffalo, Gemsbok or Beisa Oryx, Eland, Cuvier's gazelle, Dorca's Gazelle come to mind. Most of these would require expansion to house, but if such were to occur, I would hope it would be dedicated to the sahara, as Spain has a great track record for saharan hoofstock conservation using their own mini desert in Almeria, and has helped provide animals for release in their native territories.
In terms of other fauna, Bioparc could do a lot more in terms of birds, and while it has a nice aviary, there's plenty of missing avian stars of Africa like parrots or raptors. While incredibly unlikely, I would dream of one day being able to see something like a shoebill at Bioparc.
Oceanografic Valencia:
Marine fauna is far from my strongpoint, but I would love to see at some point at the park the return of a species of banded penguin, as they seem to have left the park a few years back, and where they were kept now houses sea turtles.