Gen 3,4,5

From the top of my head, Lickitung, Tangela, Magneton, Rhydon, Electabuzz, Eevee. They will get future evolutions. Could be i forgot some of them, someone else will probably fill in the rest ^^

Porygon-Z (3th evo)
Magmar
a couple of Eevee evolutions

^^ That's it, all those 8. Eevee with 3 more evolutions (grass, ice and fairy). But you'll have to wait to 4 Gen for all of them, except Fairy Eevee who is from the 6 Gen

http://bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_with_cross-generational_evolutions

Thanks for asking and for the replies. I was thinking the same thing myself. I nearly discarded a Scyther with the worst moveset a few months ago, only the fact that it had 100% IV stopped me. But I did transfer some others with very good IV's because I knew nothing about next generations.