I will try to give an answer, but it are not the most easy questions
The jaguars left in the 70/80ies, but I have no exact date (maybe somebody older than me has). The hippos left I believe in the end of the eighties (1989 or something like that), but that somebody can check in the 2007 hippo studbook or on zootierliste
2. I do not know of any plans for the area behind the lions/cheetahs, but the future plans of Burgers Zoo are a very good kept secret. I would not be surprised if nothing happens with it in the coming 10-20 years, but if then the most logical thing would be something like a Congo display, when linked to the gorillas & chimpanzees. I recently heard some rumours that the gorillas will get their own indoor house, right behind the island, but I do not know if its true, although there was some construction work (mainly sand and cutting trees) going on on that place in december
3. On a dutch forum it was recently mentioned that according to a keeper they did not know yet what the future of the elephant would be, as their exhibit could be incorporated in the new ecodisplay that will be build between the playground and entrance (although nobody seems to know what and when this ecodisplay should be build).