Which mons to save for Gen2

Tyranitar will be a new Dragonite-esque defender, and he has a double weakness to Fighting, so I'd suggest not to throw away your decent Fighting mons just in case. I haven't really thought about the rest though.

I don't think there are any Gen 1 Pokemon that gain new evolutions that are useless, but will depend on their movesets.

If you're looking for certain Pokemon that gain branching evolutions, you may want to evolve them now. Those Pokemon are Poliwrath, Eeevee, Gloom, and Slowpoke.

For example if you really want a Poliwrath with Bubble, you may want to evolve them now, because it's a 50% chance to get Bubble.

If they don't change the evolution costs, and they make branched evolutions random, then the chance of getting a Bubble Poliwrath becomes 25%, because you have a chance to get Politoed.

I think that Horsea is going to be the best Pokemon that gains a new evolution in Gen 2. Horsea has a 3rd stage evolution in Kingdra, which gains Dragon typing, so it has double resistance to fire and water, and the two types cover each others weaknesses.

Dragon is weak to ice, but water is resistant to ice, so they cancel out. The same also applies to Kingdra's water typing - water is weak to grass and electric, but dragon is resistant to both.

Seadra already has Dragon Breath as a quick move and doesn't have Dragon typing, so we could expect Kingdra to also have it, and it'll also gain STAB, so I think it'll be a good neutral attacker.

I'm hoping to finally have a use for my large Onix collection, the handful of Porygon I've caught, the Scythers I've harvested from a timely local nest and obviously Seadra.

Save Zubat/golbat for Crobat
Oddish/gloom for Bellossom
Poliwag/poliwhirl for Politoed
Slowpoke for Slowking
Onix for Steelix
Porygon for Porygon2
Chansey for Blissey
Horsea/seadra for Kingdra
Scyther for Scizor
and Eevee for Umbreon/espeon