I dislike the idea of trading to evolve itself, none of my friends play Pokemon, because them retards think it's a kids' game but anyway trading with strangers could be useful, except that I don't fully trust anyone, also it may get a bit difficult to find a trader who's active in my timezone AND has a good wi-fi connection...
I like Gengar, Alkazam, Golem, Porygon2 & Politoed just to name a few, I only had them when I traded with myself by making a copy of my ROM & using it in Gold, it was quite the headache finding the appropriate program too :/
Getting the hold-while-trading items can be a pain sometimes & it will become even more painful as more games are released, some of them are to be received so late in the game or post-E4 which is kinda stupid imo.
A better alternative would be evolving while holding items instead of trading while holding them, but without the night-time/day-time clause, & ofc make the items cheap & easily obtainable.
Another thing that would be useful is NPC trades, why trade a Ralts for a Seedot when you can trade a Graveler for a Haunter? Trading NPCs should exist more & in the same time have better offers.
-More off-topic talk in the spoiler-
It would be perfect if we can get evos by using Stones or evolving while holding items & get pre-evos by simple breeding, simple trades aren't bad either.