We're still not sure is Magearna will be a standard legendary or an event-exclusive Mythical. If gen 6 was anything to go by, the Mythicals won't have any sort of lore (at least in-game), which is why I'd like a classic (non-event) trio--they have a better chance of being given some sort of sub-plot to add to the narrative and overall content of the games.
Outside of the mascots, the only legendaries we got in XY were Zygarde and the Mythicals; I wasn't a huge fan of that so I hope it isn't repeated in SM.
Magearna has been stated to be a mythical so knowing me it won't really have any lore ingame. I do agree with you about the XY legendaries, there was really no legendaries apart from the mascots and that had a lot of disappointment factor. For SM we need more non-mythical legendaries and I'll be fine with it.