Regarding the theory and discussion about the Starters having split forms/Z-forms or not, I have something new to suggest. First off, it was said that the secret of Rockruff isn't the same exact one as the Starters no? Well, what if the following was true:
The starters have split forms, but they aren't triggered in the same way as Rockruff. In other words they don't have version exclusive forms like Rockruff does. Instead, to trigger these different evolutions, what if they need to hold certain starter exclusive Z-Crystals instead, and be leveled up at day or night, thus triggering the ability to evolve into a different form depending on their Z-Crystal they hold? Now, the leaked evos we have seen, may or may not be part of this/ or they may be the original natural evolutions instead, whereas the Z-Crystal method, if true, may allow players to actively decide their final form, while in addition granting the starters their own signature Z-Move.
The basis of this theory goes back to Rockruff, who as we know, is Rock type, and the Z-Crystals, or crystals in general, are minerals. In short, could it be that the starter secret is a Z-Crystal triggered evolution method, that in addition may be triggered by day/night or not. This may explain why the starters were featured in the same page as Rockruff stating they had secrets too, but didn't have/share the exact same method as people pointed out.
Now, given, it could be a limited number of Z-Crystals, or just certain special ones designed solely for the Alola starters to use for evolution. This could kind of, maybe, justify the commonly speculated term "Z-Forms" that people have been throwing around lately. It also adresses the concern for people who hate Version Exclusive Evolutions and want full control of this aspect. Granted, Z-Crystals can still be version exclusive. Take it as how you will.
Once more, this is just speculation, and not fact, but what does everyone else think about this possibility? Is it manageable?
In a simpler form, here is what I mean ultimately:
1. Each of the starters has their default final evolutions, which may be or not be the leaked concept arts and is reached by mere level up, and is meant for people who don't want to go through the trouble. This requires nothing special.
2. Alternate Z Evolution Forms - which is triggered by holding a Z-Crystal in either of the two following methods described below (may be more.)
Method 1:
They each have a special exclusive Z-Crystal that triggers a different Evolution form after level up and is different from their default evolutions.
Method 2:
They each have a special exclusive Z-Crystal that triggers a different Evolution form after level up,and is different from their default evolutions. In addition, this method is also influenced by day or night time.
Note, it can be one of the two methods, not both.