I think before I start with my suggestions/ideas I need to make the point that in no way do I think Marwell is a bad zoo. I just think there are a few major points that need addressing (some of these will overlap with NMM) :
The African Valley
This has been discussed to wit's end, and as NMM has said previously, its a huge task to deal with, and I don't think it's necessarily the easiest space to develop considering its terrain. I think as it is currently, with the Prezwalskis Horse, it is the best use of it they currently can. I think that despite the difficulties in developing the land, it needs to be sectioned off into individual exhibits in the long term, rather than using it as a large field that is not meeting its potential. In the short term, the biggest complaint is a lack of viewability, could moving the large outdoor shelter/tent would allow for more viewing of the current inhabitants?
The Collection Itself
Marwell is very clearly specialised to Ungulates, and I personally love this and think it is a great thing. However, the actual populations of a lot of these species are *very* low. Lesser Kudu, Arabian Oryx, Beisa Oryx, Roan Antelope, Lowland Anoa & Giraffes are all at 3 or less, and, especially with the Giraffes and Roan Antelope, cannot fill the space they are given, In the last 2 years I've visited, the main field (giraffes, zebras & roan antelope) has been practically empty, with the odd zebra right at the back.
But its not just the ungulates, All lemur species are at under 2 individuals, and "common" species like the Meerkats, Red Panda & Binturong are at low numbers too, making the zoo feel very "empty". In general. the area between the Giraffes and the Clouded Leopards is the most lacking in numbers and species, alongside the back end of the zoo.
Old/Unused Enclosures
Walking through Marwell can sometimes feel like a ghost town, especially at the back end of the zoo and across south road. With the Macaque Island, GLT Walkthrough, old Sitatunga Enclosure, old Red Panda Exhibit & old Meerkat Exhibit at the back, remnants of enclosures on South Road, The old Warthog Exhibit and old Rhino Housing are all empty.
I think that this is one of the major things that Marwell need to do! Old Meerkat Exhibit, Sitatunga, Rhino Housing and possibly even the old Red Pandas wouldn't require tons of money or resources to fit for new inhabitants (Another Mongoose Species or Porcupines?, Kudu could be moved to old sitatunga exhibit? Rhino housing could hold a swine like Warthogs or Wild Boar and Tree Porcupine in the Red Panda?).
Warthogs, Macaques and GLT walkthrough are a different story... but something like Aardvarks could work in the Warthogs & Macaque island -with some security added - Mantled Guerza?
Smaller Species
Despite Marwell having a clear lack of certain animal groups, I do not believe that it necessarily means they can never be a good zoo! Cotswold Wildlife Park, for example, has no bear or great ape species, however is widely considered to be a very good collection!
It would be quite easy for Marwell to encourage smaller species into already used sections of the zoo: The old Nile Monitor exhibit, Tropical House itself and the room connecting to the Crocodile Monitors (possibly tamandua in the main enclosure? and aye ayes or slow loris in the connection?) and the Fur Feathers & Scales buildings could probably accommodate some species, e.g. small primates such as Tamarins & Marmosets, or some birds like Kookaburras.
Empty Space
There are a few locations that are not utilised at all at Marwell, most notably the area opposite the Giraffe house, that previously contained the Marabou Mansions, and the area between the entrance and Gift Shop (as well as South Road, however they are choosing not to use that) By utilising this space it would allow for the first sections of the zoo to feel much more alive then they currently do.
Finally, Utilisation of Enclosures
Marwell, in my opinion, struggle with the way they utilise their enclosures. AS NMM mentioned, the Banteng would work significantly better in in the paddock behind the "Eco Island" due to the sheer size of their group! Additionally, I think that the Lemurs worked significantly better in Life Among the Trees! The current primates simply aren't as active in the space as the Lemurs were, and the Lemur Walkthrough is... yeah... would work much better for another species (Secretary Bird? Diana Monkey again?). I also think that the Ostrich should be moved to one of the multi-animal paddocks- they are currently tucked away in the back corner opposite the tigers with only 2 individuals! This could allow for that paddock to be used better (could maybe hold a rarer species like Hyena? or more hoof stock such as Greater Kudu/Hartebeest?). Positioning the Blesbok in their current location doesn't make much sense, as without the Leopard Walkway there is a very limited visibility - could they work better over the far end of the zoo where the Banteng are now?
However, as NMM pointed out they have been doing a lot right. The Siamangs, Wild Explorers, Sitatunga, Snow Leopards, Okapis, Clouded Leopards and Tropical House are all amazing exhibits and are part of the reason for my interest in Zoo Architecture. Marwell also have amazing records with breeding and conservation, being majorly involved with the Scimitar Horned Oryx releases!
I hope this was a decently constructed list of Ideas and I'm curious to see what others think.