1) With the Myth&Magic system, will we see magical auras? Like the Surrounding system, an area might also be associated with a Myth, and exhibit properties of that Myth. Let's say the Myth generation creates a connection between ice, lizardmen and silver (I will call these spheres), and an area highly aligned with this Myth would be colder, show often snow, ice formations, lizard-like creatures, abundant silver veins, silver colored trees, and in general entities associated with those spheres.
The current good/evil biome system is planned to sooner or later be changed out for a more generalized Sphere-based biome system, so yeah, something like this could probably happen. None of these are actual Spheres though, so they'd have to be connected to one in that case (Like DUSK or MOON).
I think this is discussed in one of the DF talks.
2) Again on this aura concept. Will we see the properties of an area modify the value of its surrounding? E.g. a fortress with a shrine to a God: will the influence of that God increase with the prayers? Will we see anything like of building a fortress in an evil biome and trying to create and expand an aura that opposes the evil biome and having the evil biome effects less strong closer to the church?
Always on this, let's say there's an area with a verdant pine forest and full of bears, and the Myth generation creates a connection between a lightning god, bears and pine trees. Given how the area "looks like" that Myth, will the association of this area with this Myth increase over time, and see the place get more lightning storm, flowers that give you a shock if you eat them, then magical creatures of lightning and so on?
The myth gen is not actually "myth" with any metaphysical properties, but really just very early history that includes the creation of the universe. In low-fastasy settings the myth gen might be actual myth, but then that just means that it didn't really happen (and the world was created my natural real-life processes).
Biomes changing I'm pretty sure will be a thing though.
4) More in general, will Myth creation stop at worldgen? Or will it also run during playing time and include player introduce elements?
Myth gen will end, but myth gen events will still be possible, if rare. Some good examples can be found in the Tales Foretold story (
http://www.bay12games.com/dwarves/story/tt_tales_foretold.html), with various mass summonings of critters, ascensions to godhood, and the like.
5) How much can we expect in terms of procedurally generated magical content? Will it include normal and widespread creatures and plants? Will we have generate worlds with large forest of procedurally generated trees, animals, stone, metal etc...?
Depends on what you put the planned random slider at, with the lowest setting having none, and the highest setting having pretty much only random stuff.
3) is interesting though, placebo magic is something pretty cool after all.