Among all the itmes we have now only Sun Stone has some more pokemon to evlove with in future genrations:
Gloom into Bellossom
Sunkern into Sunflora
Cottonee into Whimsicott
Petilil into Lilligant
Helioptile into Heliolisk
Talking about impact, in Gen IV we will have tons of new evoutions (like 25) for Get I-III mons: Murkrow, Piloswine, Misdreavus, Yanma are in that list
I think after you evolve all the mons with items you should keep them. I think someday we will be able to trade them.
Should we be saving evolution items for later generations?
Arguably the only semi-useful pokemon that evolve using evolution items in gen 2 are Scizzor and slowking. Once you evolve these pokemon and get the pokedex entries, should we start stockpiling the items? Can they be used to evolve different pokemon in later generations? Do they have effects in later generations? I saw that Kings rock can make the pokemon youre fighting against more likely flinch. On the other hand, items like up grade i think might be completely useless. they can only evolve porygon and in later gens dont do anything else. Anyone have a good understanding of the current gen 2 item's impact in later generations? also, do you think we might be able to trade evolution items in the future?
Answers
Based on this answer, I might save a few sun stone only. I think that these items take up some of your item slots, and I don't really want a lot of my item slots filled with them on the chance that we might be able to trade items someday, or they might eventually be useful in another way.
Sun stone will also be useful for evolving additional sunkerns, since there is no other path or evolutions for them, and here they are fairly common.