I've long been fascinated by the idea of more than one sentient species coexisting on a planet at once. And it's not even that far-fetched. Homo sapiens and Neanderthals coexisted for a time. It'd be neat to find out what might have happened if Neanderthals had continued to exist.
One of my own conworlds, Traipah, has three sentient species on it: Ah'Koi Bahnis, Duenicallo, and Shaokennah. Four, if you count the Zaen, which are symbiotes that about 90% of Ah'Koi Bahnis have. But I don't usually, since the Zaen can't reproduce on their own. Oh, and I've been contemplating adding another sentient species to that planet, a species of sentient planimals. (Planimal = a plant that can move around like an animal, and needs to eat animals to get certain nutrients. Kind of like carnivorous plants like Earth has, but cooler.)
(For those who don't know, "Enterprise" is a Star Trek prequel series that focuses on the first warp 5 Earth vessel, the Enterprise, captained by Captain Archer. Time travel features in the first episode, things get weird, then they get even weirder halfway through the series. If kengr is right, the last episode will retcon the entire series. Like that season of Dallas that was retconned by it being a dream of a comatose kid, but a whole series, not just one season.)
Edited to add: I just remembered, not only does my Traipah planet have 3 different sentient species on it, the Ah'Koi Bahnis also have a bunch of radically different races/subspecies. Before I'd even heard of Enterprise (Hell, before Enterprise even aired), I had in their history tales of a tree-dwelling race of Ah'Koi Bahnis (named Mahg'lih'mihn) that went extinct. Then there's two different water-dwelling AKB races, one freshwater and the other seawater. And a race that are exceptionally tall, as well as another race who look like children (and notoriously grumpy about it). And the historically dominant race of AKB, called the Ihg'Dahk, went extinct during The Reformation and gave rise to the Tiir'Gihn. Lastly, genetic engineering created the Yain'gah and the Chahm'ah'kaag. Yain'gah are rather more humanlike than any other AKB race, and the Chahm'ah'kaag have skin that changes color like cuttlefish skin.