Let me state this before Barinellos sees this and loses his cool. Marit Lage is not an Eldrazi. I don't know what she is, but definitely not an Eldrazi.

Because there are only three. Also because why would there be an Eldrazi sorceress? Also because there's no reason for her to be one other than tentacles, and not all the Eldrazi even have tentacles as their most important feature. Like... it makes just as much sense to say that she's a giant octopus--you could, but there's no particular reason to think that "sorceress that became a tentacle monster"="she was an octopus and then was even moreso an octopus."

There are only 3 _known_ Eldrazi, and probably only three true original ones sealed on Zendikar (which the others in the game are all spawned off), but that does not rule out the possibility of there being others in the infinite multiverse. I could be wrong if there is a specific reference in the lore somewhere of course, that I can't recall or find. In fact, it's entirely possible that Marit Lage was sealed away in ice prior to the 3 Eldrazi being sealed on Zendikar, and that Marit Lage is the same type of being, although she doesn't have to be, and there is no indication that she is, especially since her token is Avatar type and black colored, if I recall correctly.

Eldrazi seems to be a type of being native to the blind eternities which feed off of reality of planes, particularly their magic, so I'd guess that there could be more out there, just ones that weren't found and caught by Sorin and Company, feeding on distant unknown planes.

Well if they decide they need a new major event they can make her eldrazi if they want

screw the idea of Merit Lage being an eldrazi, think about if she is a planeswalker! like a planeswalker of such power they can manifest as a giant kraken like creature? am i the only one that thinks that maybe lage as a planeswalker would be such a turning point in expectations of planeswalkers that she would basically be the next Ugin.