With regard to how dragons are better for Tarkir, the possible answers seem to be ecological and aesthetic. Ecological because having a predator controls the human population, and resources are harnessed more sustainably. This is evidenced by the difference in art of the dual lands from Khans and Fate Reforged - the Khans dual lands are way more damaged. Aesthetically, fighting against a much stronger enemy is noble, and is presumably preferable to fighting other humans.
Well generally they've been good at letting the creative team do their work but also making sure that the story and lore work with the mechanics, so I am quite optimistic.
I'm not sure you can speak of any kind of "biology" if they aren't even made of physical matter, but I guess it's a matter of semantics.
Biology isn't limited to the physical, some scientists would even define things like cellular automata as life, albeit digital. Anyway, I think if Nicol Bolas wanted to consume the Eldrazi he would have just gone to Zendikar and done so. If he wasn't powerful enough to do so yet, then he wouldn't have released them at that point in time. He probably wants to direct them towards some planes and destroy those planes, or just occupy all the other powerful planeswalkers to deal with the Eldrazi while he executes his true masterplan.
They'll probably keep the timeline changes local to Tarkir because it gets too crazy otherwise. I don't think they intended this block to effectively retcon everything in Magic history, even events on Dominaria. In all likelihood, Ugin is healed and manages to keep that fact hidden from Bolas (who probably has spies everywhere), such that everything outside of Tarkir takes place as it would otherwise.
I wouldn't be surprised if Narset's planeswalker spark ignited at that point. The story was pretty vague about her exact fate, by saying her body crumpled. There could also be an alternate present after the events of Fate Reforged, so we might see a different version of Narset in Dragons of Tarkir, maybe a mono-colored one like Yasova. It would be kind of funny to see Sarkhan's reaction to new Narset who doesn't recognize him but to whom he feels indebted. Alternatively, Narset Planeswalker might planeswalk to the new timeline. That could make an awesome love triangle.
With the whole debate about Zurgo's intentions, he can certainly justify his course of action, since Sarkhan did murder his clansmates, albeit inadvertently. Perhaps we tend to see Zurgo as evil because Sarkhan is actually saving the world this time, we've known Sarkhan for much longer, and we've seen the cruel side of Zurgo. So if all three of them sat down for a chat Zurgo might decide that it would be better for the Mardu if he let Sarkhan go. Or perhaps not, since that would mean the present Mardu would be no more.
It is hard to characterize the personalities of these characters because they are often written by different people. Narset has certainly been very boring so far, but that does make sense given that she has been stuck in a monastery for many years. I'm sure she will be much more interesting once she starts planeswalking. Plus the power of her ability in EDH makes me think that she would make a good planeswalker (maybe an emblem that says 'At the beginning of your upkeep exile the top four cards of your library. You may play any nonland cards without paying their costs until end of turn.').
Biology isn't limited to the physical, some scientists would even define things like cellular automata as life, albeit digital. Anyway, I think if Nicol Bolas wanted to consume the Eldrazi he would have just gone to Zendikar and done so. If he wasn't powerful enough to do so yet, then he wouldn't have released them at that point in time. He probably wants to direct them towards some planes and destroy those planes, or just occupy all the other powerful planeswalkers to deal with the Eldrazi while he executes his true masterplan.
With the whole debate about Zurgo's intentions, he can certainly justify his course of action, since Sarkhan did murder his clansmates, albeit inadvertently. Perhaps we tend to see Zurgo as evil because Sarkhan is actually saving the world this time, we've known Sarkhan for much longer, and we've seen the cruel side of Zurgo. So if all three of them sat down for a chat Zurgo might decide that it would be better for the Mardu if he let Sarkhan go. Or perhaps not, since that would mean the present Mardu would be no more.
It is hard to characterize the personalities of these characters because they are often written by different people. Narset has certainly been very boring so far, but that does make sense given that she has been stuck in a monastery for many years. I'm sure she will be much more interesting once she starts planeswalking. Plus the power of her ability in EDH makes me think that she would make a good planeswalker (maybe an emblem that says 'At the beginning of your upkeep exile the top four cards of your library. You may play any nonland cards without paying their costs until end of turn.').