Hmm.. it's not big enough for Bongo and hogs would ruin the turf in days.
Maybe Duikers?
I remember this exhibit looking wonderful and modern when it opened (back when open-topped monkey exhibits were innovative in themselves). It's still pretty good, but CGSwans is right - it's definitely under-furnished.
It was quite muddy when I visited so RRH's are definatley a no no. If more foliage and hiding places were added then I think Duikers would look great here but with the colobus seemingly eating everything in sight then I think this would be hard to achieve.