As I said before I think it'll be a good idea to let Pokémon that evolve by stone evolve again, I mean no Pokémon that evolved by trade could evolve again until Porygon 2 could in gen 4, they can always break the "tradition" of stone evolutions not evolving further.
I see Phione as a legend too but it's official classification is still blurry...
Either way I want an evolution for Tropius that's Grass/Dragon.
Most Trade evolving Pokemon are Stage 1 to Stage 2's. Only Stage 1 Evolutions that still exist today that haven't evolved are Scizor, Steelix, Slowking (alternate evo), Huntail, Gorebyss, Milotic (Gen 5+), Escavalier, and Accelgor.
Now while Porygon can still evolve again once after being traded, it has to be traded again to evolve. Both with items. So taking that into consideration, only Scizor, Steelix, Slowking, Huntail, Gorebyss, and Milotic should be able to evolve again but only through trade since Porygon is making the rules on such evolutions.
You can even go as far as to argue that Porygon's evolutions are highly artificial as they are program upgrades. Which I know some people usually do argue this.
As far as stones go, there have been Stage 1 Stone evolvers since Generation 1 that have never been shown to have been considered for evolution. Even if they did the new evolution would have absolutly no new moves. The only point to evolving them would be for updated stats.
Arcanine, Ninetales, Simisear, Cloyster, Starmie, Simipour, Eseggutor, Simisage, Delcatty, Musharna, Whimseecot, Lilligant, Roserade, Cinccino, and Mismagius would not benefit in any way other than stat boost with evolutions. They don't learn a great deal of attacks when they evolve, I believe then most they learn that isn't a [Start] Move is 3 or 4 new attacks.
Pokemon like Honchcrow, Froslass, Sunflora are different since they retain a fairly larger level up list, Honchcrow being the least diverse with 6 new moves to its level list. Sunflora's is just a slightly altered version of Sunkern's attack list and Froslass keeping a healthly attack list it to put her on par with Glalie as an evolution. Course Glalie would have to evolve as well in a most likely case.
Still 5 generations, 4 chances to evolve past where they were, not one was ever touched. Only Trade evolution rules seemed to have changed in an argumentativeness way. While stone evolutions have remained true to their original sets through out the generations. (Eevee evolutions not listed cause there is a very very VERY small chance that they will ever evolve again.)
I do see alternate evolutions as a possibility for those that don't already have them for all those that evolve by stone since that seems to be a popular trend with a lot of the stone evolvers.