Things to keep in mind regarding "Extinction":
-Should Scarabs be unable to be summoned by Pharoahs if this card is used on them? I think yes. Can't summon an extinct creature. However...
-Should Fireflies be unable to be "summoned" by FFQ if this card is used on them? I still think so, because the FFQ can always produce offspring. [Both cards use the term "generate", which is very unspecific as to how.] Also, think about it balance-wise. Scarabs are sometimes used with Bone Walls, and using this card on them is going to be severely counter-productive as it already is. At least get the most of it. Whereas Fireflies are often used with Hope, and this card will render Hope useless (or nearly useless) regadless. Cut them some slack.
-Should Mummies be able to turn into Pharoahs with RT, even if Pharoahs are extinct? I think yes, because they are going
back in time. (They're "extinct" already, that's why they are walking corpses. This card being used on Pharoah should definitely have no effect on Mummies IMO.) Alternately,
-Should Skeletons still be able to randomly turn into an extinct creature? I think yes. Again, "going back in time" idea.
-Should Immortal creatures die if their mortal cousin has this card used on them? I think no. They're immortal. And I think it's obvious that innately Immortal creatures (like Phase Dragons) cannot become extinct.
-Can Skeletons themselves become extinct? If so, what does Boneyard do afterwards?
-If Shriekers are extinct, can Graboids still evolve into them? I say yes.
-Can or should we give

creatures immunity to extinction?
-If you chose to get rid of all creatures in people's hands and decks when they become extinct, what happens when GotP becomes extinct?
-Is Fate Egg limited to not turning into extinct creatures? I vote yes, because they're not going back into time like Skeletons are.
-Likewise, I think Nymph's Tears cannot produce extinct Nymphs.
-What happens when Malignant Cells become extinct?
I think this is a great idea for a card, but too much to think about and it'll probably be too complicated for them to ever want to add it to the game. Keep running with it if you want but it might be best to ditch the idea, unfortunately.
