Not sure if anyone has yet to speculate on this, but do you think some pokemon that do not have Alolan formes could still get midday and midnight forms?
One group of pokemon this would work well for are pokemon who evolve during the day (Roselia/Roserade, Tyrantrum, Chansey/Blissey, Lucario) and those who evolve at night (Aurorus, Chimecho, Gliscor, Weavile). Essentially they could have split forms easily implemented depending upon the version or simple a day/night system like espeon/umbreon.
Similarly, moon stone evolution pokemon like Nidoqueen, Nidoking, Clefable, Wigglytuff, Delcatty, Musharna could be evolved using a sun stone to acquire a midday form. As well as sun stone evolution pokemon attaining a midnight form using a moon stone. The same goes for Dawn/Dusk stone evolution pokemon.
I mean, if Solgaleo and Lunaala are so powerful they cause rockruff to split into forms, there is a huge chance other pokemon who have an affinity for the day/sun or night/moon will be influenced based upon the version they select, right?
(Also, after defeating the Mascot, won't balance be restored? I wonder if rockruff has access to both forms after the main story is finished since Lunaala and Solgaleo will be balanced? Will it merely have a espeon/umbreon like evolutionary method at that point? )