I think that sadly, Sylveon will be the last traditional cross-generational evolution for the foreseeable future.
GameFreak got a lot of negative feedback for evolving a bunch of old 'mons in Gen. IV. I guess the intention was to revitalize old Pokémon, make them relevant again as well as boosting their power. However, many fans thought it ruined the feel of the old Pokémon. I mean, who wants to play with Magmar if you can evolve it? Why make Dusknoir at all, it's just a boosted form of Dusclops? Who wants to settle for a Togetic when they can have Togekiss? Old Pokémon, many of them big fan favourites, used to be somewhat weak, yet decent enough for in-game runs, and great to use in less-intense competitive battles (such as the lower tiers on Smogon). However, with the addition of their evolutions, they became mere transitional forms, while the "ugly big brothers" took the fully-evolved limelight away from them. Sneasel used to be the complete and final form of its evolutionary line. Now it's just an unevolved form, to be obtained and promptly evolved into the more powerful Weavile. Electabuzz, Rhydon, Magneton - first generation favourites, yet now reduced to middle stage Pokémon in the shadow of Electivire, Rhyperior and Magnezone. They are now largely regarded as lesser versions of the fourth generation "usurpers". Myself, I like the new evolutions, but I'm trying to get across the points of the more negatively-minded parts of the fanbase.
So GameFreak hatched a plan, a compromise keeping the old 'mons as they are, yet still revitalizing them and making them more powerful (so they can be used at the forefront of competitive play): Mega Evolutions.
Mega Evolutions don't take away the spotlight of the ME'd Pokémon. For intance, Mawile is still Mawile, you won't find its "evolved" form in the Pokédex. Mawile will still be part of your team, reside in your boxes, and learn Mawile moves. It will just transform in battle, akin to getting a power boost. Yes, it will get a kickass new design, but that design will still be Mawile. Your good ol' Mawile will still look the same after the big battle. GameFreak try to recreate the power boost and new design features of an evolution, but without making the evolved Pokémon obsolete. They hope to both please the crowd that want their favourite Pokémon to be more powerful, as well as the crowd that don't want to see their favourites outclassed by a new, "ugly" design.
This also has the benefit of allowing a fourth evolution to a family, thus giving them the option to revitalize evolution families that were previously considered "untouchable". Heck, even Legendaries can be put back in the marketing spotlight. They can add new, more powerful versions of 'mons such as Charizard, Blaziken or Mewtwo, without angering the purists. They can keep the versatile and recognisable Lucario for more shenanigans, opting to only use its power "when stuff goes real" in battle. They can give old Pokémon new levels of power, to a degree previously unheard of, but the badass, new, controversial design doesn't have to stick around afterwards. Players can still carry their favourite 'mons, they don't have to learn new names or accept the old Pokémon as pre-evolutions. On paper, it's a good compromise.
Though, I must say I belong to the crowd who will miss cross-generational evolutions. I like the idea of traditional revitalization, I hugely enjoy speculations along the line of "this thing is sucky now, but if it gets to evolve in the next generation...". New evolutions take some getting used to, but when you do, you can re-catch your old favourites in new games, then train them with the goal of making them even more powerful. I liked Elecabuzz before, but I wouldn't have caught an Elekid in White2 if it weren't for the promise of Electivire somewhere down the line. I enjoy Sneasel's design, but without Weavile, it is kinda weak and kinda boring. And Porygon would be nothing without its later brethren, giving it way more power than it ever asked for (and looking quite cool to boot!).
I really hope I'm wrong in my assumptions of GameFreak's decisions. Or if I'm correct, I hope they will revoke it come Gen. VII. Farfetch'd, Dunspare, Chimecho - your time to shine will come. I'll be waiting.