I just thought of an interesting theory....
Pokemon Sun and Moon...with it comes the introduction of Alolan forms...and while there's plenty of speculation and theories as to which possible Pokemon may get Alolan forms, there's another possibility that I find rather interesting...
Which two pokemon are expected to play some sort of role within Sun and Moon? Given their ties to the Sun and Moon respectively as well as some possible hints found on Alola's region map...
Imagine Alolan Lugia and Alolan Ho-oh...XY and ORAS introduced two kinds of form changes respectively in Mega Evolution and Primal Reversionm, both of which involved legendaries...And while Alolan forms aren't necessarily a form change...it's an adaptation...
Perhaps being in Alola allowed Lugia and Ho-oh to take on different forms due to perhasp the presence of Solgaleo and Lunala or some spiritual energy or some other magic pancakes...
I don't think it's too
to think of them taking on other forms especially since one of them already has in the past...
While I don't think Lugia will become Shadow Lugia specifically, It may gain a Dark typing as a possibility, perhaps Dark/Dragon? Dark/Water? Dark/Fairy?...as for Ho-oh it's a bit more difficult to pinpoint what typing it may receive if it were to become an Alolan pokemon...The sun does produce solar energy/power which through human technology is converted into electricity so, Fire/Electric? or perhaps it holds a more natural means to converting solar energy...photosynthesis...Fire/Grass? Maybe it would gain a new ability that acts like a powered up Chloryphyll, Fire and Grass types benefit the most from Sunny Day afterall...
I feel like Alola forms may affect any sort of Pokemon, including Legends, of course not all of them will be affected but I feel some will and Lugia and Ho-oh are distinct possibilities...and while so far only Gen 1 pokemon have been revealed to get Alolan forms I doubt it's only going to be reserved for just the one generation...