Assuming the regular Eevee is even used.
I can't see Ash being defeated by something like a Leafeon or Umbreon in the last round. It just doesn't have that alpha pokemon lead status.
There's no single promoted poke like a Darkrai or Blaziken.
Eevee probably will be used. It + Kanto Trio + Johto or Sinnoh Duo works well aesthetically.
Of course they don't, but perhaps a certain something will...
Patience. The League's beginning, let alone the final battle, is still over a month away. The single promoted Pokemon shall come in due time.