If anyone has any further tidbits, hints, or fun new species that have appeared, I'd love to hear your news! Otherwise, first zoo-trip to continental Europe here I come!
From your list:
Cologne Zoo:
Red-shanked Douc Langur - easy
Tree kangaroo - easy
Musk Ox - very easy
Ring-tailed Vontsira (have seen these once before at RSCC, but would love to see them again) - easy, very active, they have indoor and outdoor enclosure parts, the outdoor have wire mesh and is difficult to take decent photos, the indoor one have glass but is darker and animals usually prefair to stay in the outdoor side
Balabac chevrotain - difficult, in deep rainforest exhibit with Great Argus and Channel-billed Cuckoo amongst others, probably you will see it passing through vegetation and not staying quiet at a point for photograph, not easy to see the whole floor from the high bridge
Hausa Genet (if on show) - off show, sorry
Rufous Elephant Shrew - impossible, they share exhibit of the sun squirrel, but they pass the day hidden
Northern Luzon Cloud Rat - easy, but very dark enclosure for photos
Red Legged Sun Squirrel (only ones in Europe on ZTL) - easy
Geoffrey’s Cat - I didn't remember seeing them
Duisburg Zoo:
Baby the River Dolphin! - easy, but early in the morning it's usually absent, check later in the coming hours
Lowland Paca - difficult, they are just opposite to the river dolphin, but usually hidden unless staff gives them the food, I was fortunate for see them just when they are being fed
Tasmanian Devils - easy, but TOO active, they run constantly through all the huge exhibit, so not easy for photos, exhibit have a low glass wall but the ground is very elevated with many logs and plants and rocks, so devils can run constantly out of sight
Wombats - almost impossible, I passed many times to the wombat and reck-necked wallaby exhibit, and not the least sign of wombats, I guess they pass the day sleeping into a den
Koalas - easy
Short Beaked Echidna - easy and active
Arctic foxes - easy, but you will see just a sleeping fur ball
Banded Mongoose - I didn't remember those
Fishing Cat - relatively easy, outdoor part of exhibit not good for photos, indoor part good, they pass many time inside the big hollow log that hang in the background, photos of entire cat maybe are not easy, but good for portraits of the head when they are inside the log
Dolphin show - I skipped it (for sure I would have seen it if they still had Commerson's dolphins!!)