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Do you think Game Freak will bother to make every single Pokemon available solely with the gen 7 games? Most previous generations managed this:
Gen 1: Mew was the only unobtainable Pokemon, it was distributed in an event.
Gen 2: Only lacked the Kanto starters, fossils, Kanto legendaries and Celebi (all distributed in events).
Gen 3: Only lacked Ho-Oh, Lugia and the mythical Pokemon, all of which were distributed via event. You could also get the first two and a number of Johto Pokemon in Colosseum/XD if you didn't want to hunt them down in Emerald/FRLG.
Gen 4: Only lacked mythicals (available via events - Phione/Manaphy were available via Pokemon Ranger) and the Regis (who were catchable in Platinum if you received the event Regigigas).
Gen 5: Technically the only generation so far with gaps. BW and BW2 lacked a number of Pokemon themselves, but you could get most of them via the Dream World or Dream Radar. Even though some of the remaining missing Pokemon were distributed in events (such as the starters and some of the legendaries), it still lacked the legendary birds, legenday beasts, Celebi, Phione/Manaphy and (for some reason) Unown.
Gen 6: All Pokemon were available either in-game except the Mythical Pokemon (which were all released via events).
Gen 7 currently has a lot of gaps, although the eShop release of RBY and GS do fill some of them (and they are compatible with gen 7's Pokemon Bank). Currently Game Freak have four options:
1) Include the missing Pokemon in USUM
2) Make the missing Pokemon available via a spin-off like Ranger/Dream World/Dream Radar
3) Distribute a shit ton of Pokemon via events
4) Not bother to fill the gaps at all
What do you think they'll do?