I'm not sure the elephant grass paddock would suit them either. Even without the cost of a new house and hard standing, the grass area is not much bigger than their old area, and has incredibly steep sides which would need sorting.
The grass area of the elephant paddock is used by them (afaik) for exercise and grazing-even if they don't have fulltime acess. So if used for something else they would be denied that freedom. As you point out, its also a 'mounded' or hilly shape with ditches- totally unsuitable for Giraffe.
Realistically, if they were to have Giraffe and /or Zebra again (they had Zebras many years ago) the place for them would be in new accomodation and paddocks in one of the undeveloped fields owned by the Zoo(there is still undeveloped land isn't there?) New housing = major expense= unlikely to happen in the short term.