@Nerdman
For the most part we have tied to stay away from fantasy, magic and mythology. There has been talks of a separate C2C Fantasy add-on however. While the "Punks" are scifi in their nature they still sit on the edge of plausibility. Mythological creatures, magic spells and other such fantasy themed things are too impossible for this and can only possibly be used in high tech situations such as nano-tech acting like magic or genetic engineering making mythological style monsters.
The whole theme of these techs is to use current technology to make advanced technology. Such as the Di Vinci Tank. It used mechanical technology, with wood, iron and even early cannons to make a tank long before there were tanks. Its that kind of application the alt-timeliens were made for.
And even the Bear Riders are based in some plausibility. Bears are real, and while extremely slow and dangerous one could selectively breed for tame bears. Eventually one could have a domesticated bear. It would take a lot longer due to their low amount of offspring (2 to 3) and aggressive nature. Honestly I think possibly black bears might be the best type of bear. They are smaller than brown bears and have a wide diet. Some even have shown social behavior with other bears.
Here are some guidelines ...
What makes an animal possible to domesticate?
The world's foremost expert on domestication is arguably UCLA Professor Jared Diamond, who is best known for his popular science books like the awesomely titled Guns, Germs, and Steel. In the book, he lays out six basic criteria that an animal has to meet in order to be possible to domesticate. Let's run through them briefly:
1. The animal needs to be able to eat a lot of different thing and be willing to live off the scraps of humans. If the animals are able to eat stuff humans can't, such as grass, then even better. This also makes strict carnivores somewhat more difficult to domesticate than other animals, as it commits humans to providing a ready food source of other animals for them to eat.
2. The animals need to grow up fast, or at least faster than humans. There's not much point in trying to domesticate extremely long-lived species like elephants or tortoises, as it can take several years before they're even remotely useful, and their long life cycle limits how quickly their numbers can be replenished.
3. The animals must be willing to breed in the close quarters of captivity. Any creature that demands a lot of open territory in order to breed - pandas and antelopes are good examples of this - are terrible domestication candidates.
4. The animals have to be naturally pleasant. An unpredictable or ill-tempered beast is just going to be dangerous to attempt to keep enclosed in a small area. It's possible to meet some animals halfway on this - for instance, the American bison can be kept in huge enclosures on ranches - but that's as close as we can get to full-on domestication for species like that.
5. It isn't just pleasantness - the animals need to be calm as well. Skittish or flighty animals will constantly attempt to escape, and it can be almost impossible to control them even if escape is impossible. This is what seems to have kept foxes from being successfully domesticated, as they're far more skittish than dogs and wolves.
6. The animals need to be willing to recognize humans as their new masters, which means they must have a flexible social hierarchy.
The remarkable thing about all this is that some animals that seem almost indistinguishable from their domesticated counterparts can prove to be incurably wild. The difference between foxes and dogs doesn't seem like very much, domestic pigs seem superficially very similar to the impossibly wild peccaries and warthogs, and zebras can even interbreed with the fully domesticated horses and donkeys...and yet outside a couple isolated cases of taming zebras, the species has proven completely impossible to domesticate.
Source:
http://io9.com/5756178/why-can-some-animals-never-be-domesticated
1. Black bears eat our garbage so check.
2. Black bears live around 18 years in the wild though some have reached 31. And they can start to have babies around 3–5 years old. That's getting to be a little long, but still shorter than a human lifespan. So maybe.
3. I don't know this. I would suspect they may need a large range.
4. Yeah they fail at this, but I would think if you could get past the first few aggressive generations you could get a tame bear population.
5. Yeah they would have to be caged well like the Russian foxes until they were more friendly to humans.
6. Do bears have a hierarchy? I don't know.
So yeah zebras and bears may be at the edge of this. But if they were easily domesticated then it would not be an atl-timeline would it?