Okay, it seems thar heirs can shapeshift into their aspect (at least if we look at John). But would, say, a Fairy class be able to pull off something similar? Like Maid of Void turning into literal gates into Void or some sort of tiny Voidy Creature. Or Maid of Hope into angel. Or do fairies simply go thru metamorphosis?

Well, the question here would be, do Heirs need to Shapeshift, and do all Fairies go through a metamorphosis? Aranea is a bit of an outlier when it comes to metamorphosis, unless you count her going Pirate-y and kind of evil a Metamorphosis on the level of Rainbowdrinkerism or Robotification + God Tier Ascension. It seems to be a trend, and a fitting one at that, but it may not be a required step. That being said? Maids have the ‘Made of’ thing, so, Made of Hope, Made of Void, I believe making themselves into an embodiment of their Aspect most definitely fits their powersets.