If Marshadow is indeed representing a eclipse, what if Marshadow wasn't the Trio Master perse, but rather, a offspring of both the Sun and/or Moon Legends of some kind, due to, as some people already mentioned, ancient myths and legends involving the Sun and Moon falling in "love"? If you think about it, it kind of makes sense. This is because both the sun and moon are astronomical objects, whereas a eclipse isn't "something physical" but rather a effect caused by said mentioned objects, mostly resulting from the Moon, which makes sense because the Moon also represents femininity, seeing as children come forth from female species.
Lunar Eclipse:
Occurs only when the Moon, the Sun and the Earth are alligned perfectly, and the moon moves into the Earth's "shadow" .
Solar Eclipse:
Occurs only when the Moon moves into the path of the Sun and blocks off all its sunlight/energy.
The name would make a lot of sense, because Marshadow (if speculation ends up true) could technically be seen as the "shadow child" of the Moon and the Sun (Solar Eclipse), or the Moon, the Sun and the Earth (Lunar Eclipse). Basically, the legendary would be the child that's born from the shadows of the Sun and Moon and/or Earth.
Besides that, I agree with others that Marshadow, whatever it may end up being, is without a doubt going to be a Dark Type of some kind, that is if it represents the Eclipse. As for its typing, I'm looking at "Dark/Grass". Why the "Grass" type you ask? Because if anyone has noticed, and I sure you have, the word "Marshadow" includes the word "Marsh" which is a type of wetland dominated by herbaceous plants. In addition, if one takes this into account, ironically all three Legendaries would represent the classic "Starter" triangle. That is if the Sun represents the Fire type and the Moon Legendary has the Water typing.
Its also very possible that Marshadow could refer to Planet Mars and its poisonous atmosphere, which in case, Dark/Poison would be the perfect ace typing for it, and make it the very first Poison-type Legendary many have been clamouring for. In addition, I'm convinced that Zygarde is going to play a bigger role as well, seeing as the Earth is needed to create the Lunar Eclipse. In essence, the line up, if the names end up being true, could end up being the following:
Solgaleo - the Sun.
Lunaala - the Moon
Marshadow - the Eclipse
Zygarde - the Earth
As for their typings, aside Zygarde who is Ground/Dragon, these are my following predictions/speculations:
Solgaleo - Fire/Fairy or Fire/Electric (the latter because the sun has a electromagnetic field around it)
Lunaala - Water/Dark or Water/Flying (the latter due to Ala meaning Wing in Spanish)
Marshadow - Dark/Grass or Poison/Dark (see above explanations)