I don't think it's quite outside the realm of possibility for there to be new cross gens in the DLC. They've already established that we can get new cross-gens mid gen with LA, introducing new ones in DLC is unprecedented, but not far off from what they've done before, they've broken crazier precedents than that.
It's possible I admit. But I'm treating as more a hope than an expectation as it would be a break in precedents but like you say not impossible.
Same to me that's the best part of both PLA and SV. Kind of jumping back and forth on a gut feeling of Eevee getting a new evolution or two.
When it comes to eevee I hope they do otherwise the fact put a rival as an eeveelution specialist in-game otherwise that was a slightly weird choice. Then again its been quite a while since they added an eeveelution so I'm less optimistic.