As the Community Day(s) upcoming are two for the first time what makes the most sense is one day to be specific to Leafeon and the other to Glaceon.
When Eevee was announced instead of a 'mon like Cyndaquil/Tatodile/Chikorita or Mudkip/Aron/Beldum the community as a whole let out a collective yawn.
Most trainers have sufficient Eevee candy and have likely been storing a few choice ones for later upgrades. An additional move only means they would pick a time to evolve them for it at their location. There is no need to go and catch them whatsoever (save a shiny variant).
However, if the rules change up for evolutions to evolve on one day to Leafeon and the other Glaceon with a specific move there’s added incentive. Further, if the only guarantee of either of them save naming for one is on the community day the incentive is far greater.
The Community day could do this while including the current 5 possible evolutions with special moves for the other Eevees. Such as, instead of a guarantee Leafeon or Glaceon would be a percentage chance of 1 in 6 or so.
So, watch for a Gen4 chance as hinted at by Niantic to be upcoming this August Community Day! Take care, trainers!