I now that Floresiensis, being evolved directly from Homo Erectus, would probably be impossible to interbreed with. I think that a good way to preserve Neanderthals would be to have a large amount of ice age fauna survive in Europe. For other hominids these animals would be too dangerous to live near.
I also think that so little is known about Denisovans that we can use a creative liscense to fill in the gaps.
What's to keep Sapiens from killing them all like OTL, either through disease, hunting, or competition for food? The reasons why Sapiens wasn't compatible with Ice Age fauna wasn't because they were too deadly for us to handle, there are lions, elephants, and other megafauna in Africa (and there was in the Middle East until recently), it's that we were too deadly for them. Much of Europe's mega fauna continued to exist for the 20,000 years we coexisted with them, until around 10,000 BC when the climate changed. We caused a new pressure that, on top of climate change, they weren't able to cope with after the Younger Dryas.