Do you have any thoughts on the difference between cards that outright say "win the game" vs cards that effectively win the game (I.E. Felidar Sovereign vs Aetherflux Reservoir, Door to Nothingness vs Sawnspire of Ulamog, etc.). I've always been partial to the latter. Thanks : )

R&D, as a whole, also leans towards the latter.

Is 1 alternate lose/win condition per set going to be the norm going forward, after Demonic pact in Origins, Felidar Sovereign in BFZ, Hedron Alignment in OGW, and Triskaidekaphobia in SOI, or is this just an anomaly?

We tend to do no more than one, unless it’s a mechanic, as a default, but we don’t always have one.

Is there a decent chance of making occasional splashy rare/mythic unicorns in the future? I feel they are done a great disservice generally. Whenever I look at Felidar Sovereign for example, I think it would have made a sweet unicorn.

I agree that we’ve underserved Unicorns.

Were Felidar Sovereign and Dragonmaster Outcast added late in development, or is their having the same art as their original printing an aesthetic thing (perhaps because we won't be getting same-art reprints in core sets ever again)?

With the absence of the core sets, we are looking for places where beautiful art can return. The current plan is if the old art is very good and makes sense in the new set we will consider reprinting the art.

Felidar Sovereign has the same art as its original printing, despite this being a new expert expansion. Is this something we should expect more of in the future, now that core sets are gone?

Yes, one of the things we realized with the core set leaving was there was no longer a way for us to reprint cards with art we liked. So we decided that if the old art makes sense in the creative of the world it’s being reprinted in that we will start giving ourselves the option of reusing the art.