Currently, it seems planned that worldgen will go through two stages: "creation"(mythgen) and then "history"(current history generation). Are there plans to possibly add an in-between stage for ancient, poorly recorded history before "Year 1" for historical events that happened in the remote past? To elaborate, this would be things like "around 7000 years ago, The Hammers of Dwarfiness was founded by Urist McLegendary" or "around 3500 years ago, the legendary dragon [DragonName] was slain by the human hero [HeroName]". Right now it would be infeasible to generate over 5000 years of proper history(few people have the PC or patience for it), and there's not much distinction between proper, well-recorded historical events(e.g. a major battle that happened just 54 years ago) and vague, legend-sy events that happened possibly before even writing existed(e.g. the founding of a civilization).
So, is there some sort of system planned to support this? It would really help with worlds having a feeling of ancientness and tying "creation"(mythgen) and well-recorded history in a more logical way.
While the idea seems attractive, there are a bunch of problems. In general, the underlying issue is that DF tries to be a simulation wherever it can; what you are effectively asking for is an even "lower res" mode for historical simulation than the existing history generation. This is not impossible, but much harder than you think, and it's far from clear whether it would be a good use of time. (Or, you are asking for some sort of "random pre-historical event table", which causes more problems than you realize and is not generally the way DF works.)
Just to work with your example for a bit... How does it know whether there are humans or dragons in the first place in that time period, and whether it is reasonable for them to be interacting? What sort of language to give the name(s) in? There are all sorts of ecological, cultural, and logistical implications which you can't just sweep under the table in something like DF.
To look at your question in another light, one reason you play dwarves in DF is that you are here to *create* the history. Dwarves turn mountains into ant-warrens with alarming speed, dig up precious materials, make weird crafts and crazy weapons out of them, guard them with elaborate traps, create incomprehensible mega-projects, and finally delve too deep and doom themselves. In fortress mode, you *are* the history-makers, the legend-creators, the people about whom the more sensible, human adventures from more mundane times thousands of years hence ask "How did they do that? And more importantly, why?" You stand at the dawn of time, and choose whether you start with building the great pyramids, delving the mines of moria, collecting beasts from the world over for your colosseum, turning an inhospitable desert into the gardens of babylon, or whatever project you can dream up; in the almost unimaginably distant future, your creations will be the seven ancient wonders of a world shaped from the very beginning by your will. Or, you known, you end up killed off by zombie penguins the first nightfall, as your novice miner asks "why is the ground under the cart all full of water?"
What we really want / need is fully restartable history. We have been getting closer in the last few years, as the world has "come alive", but in the long term what you are looking for would be most appropriately handled in DF by a grand sort of meta-succession fort. Imagine a world that is played for a few seasons to a few decades by each person, they retire, press the "pass 1d6 hundred years under computer control" button, and then send on to the next player. After a few (or few dozen!) iterations, you've got all the mysterious artifacts, rumored but poorly-documented dungeons, ancient evils, partially-jammed but still insidious trap corridors, heroic last stands, dramatic beast-slaying stories, and thousands of years of badly recorded history to be puzzled out of the remaining engraved walls and bizarre statues that any adventurer would want.