(Darn, looks like my original reply got lost....)
As far as I know, it was the domestication of cattle that enabled the first sedentary communities by allowing more efficient plowing, irrigation, etc. That was the biggest hurdle before farming became really competitive with foraging. Besides the sheep and pig, pretty much everything we think of as 'domesticated' happened after that point. Wikipedia has a pretty helpful list:
https://en.wikipedia.org/wiki/List_of_domesticated_animals
That said, that list relies on a very specific definition of 'domestication,' particularly related to physical or even genetic changes that occur as a result of close contact with humanity. The predecessors to these animals were tamed and herded for many thousands of years before the 'official' moment of genetic drift.
In game terms, I'd recommend moving canine domestication much earlier and keeping the same 'herd' mechanic as is currently used. I'm not sure about the idea of moving all domestication events to the same tech, though.
For instance, given that domesticated cattle was perhaps the primary driving force behind the move to sedentary living, I'd recommend either making it a separate tech (and direct pre-requisite for advancing to the ancient era) or making the NW itself a pre-req for moving on. One possibility would be to treat it like the Apollo Program wonder in vanilla Civ, where only one player has to build it for everyone to reap the benefit (or perhaps just make it much cheaper to domesticate cattle after that point).
Parallel to this, I'd also suggest making each animal domestication event an actual in-game event -- whoever gets there first receives a particular benefit, ranging from the major (perhaps a free settler if you're the first to get domesticated cattle) to the minor (a slight boost to disease preventation if you're the first to domesticate cats?)
Besides treating cattle properly, I think the next biggest thing would be ensuring that the animals are 'staggered.' Perhaps each animal domestication wonder could have two tech pre-reqs, one being the generic Domestication tech, the other pushed further down the line in the ancient era.
I do like the diverse possibilities related to 'megafauna domestication' and the alt-history content, so I'd encourage you not to trim things down too far. If anything, I'd suggest expanding the role of other domesticates -- perhaps separate 'waterfowl' from generic 'poultry,' for instance. As for the megafauna, since they are alt-history, there are obviously bigger obstacles to overcome, but I really think there should be bigger benefits to doing so as well, and not just slightly better military units and specialized workers. I'd like to see something comparable to what you have with the clockpunk, steampunk, and the other alt-history content.