Evolutions for seviper and zangoose would be great candidates for fighting/normal and fighting/poison types.
I feel that there should be some pokemon introduced to explain luvdisc and alomomola.
as for single and two stage mons, I kinda hope they leave many gen 1 and some gen 2 pokemon lines alone. They definitely need to leave some pokemon as single stage from those gens. I feel that I would welcome most evos/pre-evos for pokemon from gen 3 and on. Spinda could get a ying-yang based evolution, and I think that an evo for manectric sounds awesome. To go along with a medicham evo, I think that a Hariyama evo would be pretty funny. Mawile might be neat with an evo as well, and I think that castform might need an evo or some more buff to its ability, like a sandstorm form.