I think it would work very well without the gorillas, if a big area was set aside for colobus, guenons, bongo, sitatunga, red river hog, congo buffalo and pygmy hippo. Would be especially nice if it could be viewed from a central pavillion with a 360 degrees view.
There are certain possible risks considering this idea.
-bongo & sitatunga hybridisation
-pygmy hippos can be sometimes pretty nasty and even fatal to larger animals, though the combination bongo (addax)-pygmy hippo seemed to have worked fine in f.e. Mountain View
-same goes for congo buffalo; RRHs can also be quite a source of trouble-and would turn the exhibit pretty soon into a mud field
There could be similar yet zoogeographically even more interesting combinations:
-bongo, King colobus, duiker (wishful thinking: zebra duiker), Roloway monkey and maybe pygmy hippo
-bongo (maybe just a single bull), pygmy hippo, serval
Though the latter version sounds absurd, please feel free to look at
Fuengirola Zoo in Spain and keep in mind that f.e. Tierpark Nadermann keeps servals quite successfully together with various ungulates, among them eland.
I think you asked about red pandas with mountain goats or takin. I'm sure there is a zoo that keeps red pandas with pheasants and muntjac. There could be some good Himalayan exhibits, such as cranes, muntjac, serow, takin and snub-nosed monkey or markhor, yak and bharal.
-Tierpark Görlitz had a red panda / White-eared Pheasant/ Reeve's muntjac mix; they had to take the pheasants out when the pandas found out that they're a nice midnight dinner... The often observed Red Panda -Reeve's muntjac combination (f.e. Nuremberg, Zürich) was in some cases seperated again as the pandas started to attack muntjac fawns
-Cranes & monkeys as well the other combination mentioned could be a little risky; the takin & macaque & goral(?) exhibit in Rotterdam was, according to my knowledge, also not that successful on the long run due to interspecific aggression. And the snub-nosed monkeys are at the moment so precious and rare (as actually no non-Chinese zoo besides Zoorasia and soon LA keep one species-
Rhinopithecus roxellana) that most zoos wouldn't experiment with them if they had them (even though the idea of Golden Snub-Nosed Monkey AND Golden Takin in one exhibit sounds awesome

)
@nicholas: Most South American bat species kept in zoos are a few Phyllostomidae species, which, with the exception of the huge Spectral Bat (actually at the moment not kept in any zoos to my knowledge ) and maybe the Greater spear-nosed bat, are small enough to fall prey to said owl monkeys. Therefore I would be careful in trying such a mix; especially not in the case of the vampire bats...
