Do you worry that some things that are currently regarded as spoilers/secrets may be spoiled for new players by the myth generator/viewer?6b. Time travel beyond passive witnessing of past event would be a mess. Time speed manipulation is probably more of an issue of whether the results would be interesting and worth the effort required (all timers would somehow have to be set to a different rate, for instance).
8. My understanding is that mythgen would generate the constraints for the magic systems as part of the process such that each system gets a common set of underlying principles/rules. However, there may be more than one magic system in the same world, so you might have bloodline magic, trained magic, species magic, and "divine" magic in the same world but with different constraints for each set (and there's nothing to stop e.g. two bloodlines to appear within the same world with different constraints, or have multiple panthea with different sets of rules for "divine" magic).
6b:
Perhaps, but I don't think it's so impossible. In worldgen and the unloaded world I feel like it could work similarly to destiny/fate, where some creature or object appears out of "a time portal" or whatever (with some made up memories/personality), and then history gen conspires to make sure that this creature is born (or object is made) and lives up until they "go back to the past" in the state they appeared in.
It probably wouldn't be possible for the player to do though, unless... Toady has said that in some magic update (not first pass), there would be 2-way travel between dimensions/planes, so maybe certain moments of the past could be frozen as a "dimension" code-wise and allowed to be visited? For sure this would mean time travel cannot just be a causal spell probably, and there would have to be specific points you're "allowed" to travel to, since the game would need to save a snapshot of the world to return to that point.
As for future travel, that would be functionally similar to what Toady has said about "resuming worldgen", or whatever that waiting period is after embarking where a week or so passes on the calendar, just on a longer scale.
Time speed manipulation... Yeah, it's a matter of being worth it I guess (though "interesting results", IMO is a given), but I'm curious what the challenges are and if it's being considered. One challenge for sure is that NPC's speeding up time for themselves (or stopping it for the world) would require solving "letting creatures move multiple tiles in 1 tick", and "let them do many more actions in 1 tick". By comparison, slowing down the clocks of other things/the world strikes me as much easier to solve in comparison (and since an adventure-mode player speeding themselves up would manifest as everything else slowing down, that would be easier to implement than letting NPC's do it).
8. I see; I hope then that mythgen is able to frequently make worlds with global constraints across all magic, because the mythgen demos I've seen don't give that impression to me (even within a single magic system of a single species there looks to be little in common between effects and costs, sometimes being a single word and other times requiring flesh).