Niantic is not responsible for the creation of new Pokemon. Whether we see more evolutions for the different types is dependent on if GameFreak creates more in future generations (Gen 8+). There is zero way of knowing if we'll see more or not.
Between the two from Gen 4 they'll be roughly as relevant, with odds favoring Glaceon a little more. Glaceon is guaranteed to get a good moveset assuming they stick to the monotype movesets they've repeated for the current Eevee evolutions. Even if Mamoswine and/or Weavile end up better than it it's better than any ice type currently in game.
Leafeon will also be good but faces a lot more competition from the grass-type department. It won't be as good as FP Venusaur and faces challenges from newcomers Tangrowth and potentially Roserade as well. Grass type as a whole faces competition from electric and water types as well, only being favored when a double weakness is present.
At the end of the day these two will likely fall into the role of every other Eeveelution (sans Umbreon); fantastic easy to obtain budget options that will fall short to more premier specialists.
As for your hundo, if you plan to save it it'll probably come down to immediate needs; if a grass type attacker is needed first go with Leafeon. If ice, then Glaceon. Alternatively Espeon is a solid choice, though with Abra being everywhere for the event it might not be the most sound choice at the moment.
