I think you guys are missing something with the "dual type or single type mascots" thing, which Ive already mentioned.
(I ignore gen I as there wasnt yet a plot and legend involved and all that just the freakin starters on the covers, nothing thought through yet, they just had to put something on)
Every mascot so far was either:
-single type (Ground, Water)
-+Flying (Psychic, Fire, Dragon)
-+Dragon (Steel, Water, Ghost, Fire, Electric, Ice)
(when you look whats missing: Normal, Grass, Fighting, Poison, Bug, Rock, Dark - I find it extremely surprising Grass hasnt seen the light yet.)
Dragon was added for epicness or because they ARE dragons
And Flying was added because it freakin flies
These are JUST COMMON SENSE DUAL TYPES, not "jesus christ its Psychic AND Electric holy ****."
Just as a mascot resembeling an insect would have been Bug!
From that view:
NO mascot was really a dual type pokemon.
They never have more than one "elemental" (for a lack of better word) type.
Therefore my reasoning for them being more likely again "pure" types:
-Grass
-Dark/Flying
(gamefreak wouldn't throw a fancy second type on Xerneas to make up for Yveltal being one, because the Flying part is just the common sense part, not the "oh my god its dual type" thing)
I'm not saying they would never make fancy dual types for legends, just probably not as long as there is still simple stuff to milk like.... pure Grass LOL