Ashiok, Nightmare Weaver doesn't extend beyond the limit of the card frame like other 'walkers do.

I wonder if that is implying his relationship to the other powers on the plane. You know, since part of the reason 'walkers transcend their frame is to show their power in relation to... pretty much everything else. Considering Ashiok appears next to literal gods, it could imply something like that.

I'm inclined to say that they couldn't do the same with Elspeth because she's already broken the border once. (But also I would need to look at the set again to remind myself what her card looks like.)

I'm still saying Theros is a coma dream of Elspeth. Why is Ashiok self-contained? Because Ashiok is weaving Elspeth's nightmare, naturally.

Elspeth, Sun's Champion and Xenagos, the Reveler both break frame (Elspeth's cloak on the right, Xenagos's horns at the top), so if this is Elspeth's dream, did Xenagos somehow 'walk into a dreamscape? LordLuna's theory is interesting, though we don't really know enough about Ashiok to tell how it regards the gods: Elspeth treats the gods with reverence but has broken frame before while Xenagos doesn't care for them.

And MaRo's recent post suggest that Ashiok is another one of the "Limbo Walkers": apparently there are Neowalkers created that are bound to have no link to the story and are just created mainly for gameplay purposes.

Vraska, Domri and Tamiyo are apparently this kind as well.

Is it possible that the art was repurposed and there was nothing tbeyond what we see to go beyond the frame? Or maybe they wanted his smoke-head thing to break the frame but then it would have to obscure the text? I guess they could always cut it off, but maybe it didn't look very good?

A lot of that smoke would have been really hard to do. Smoke has a natural transparency that would have made it difficult to break frame where other things just block out the overlap completely.

Oh god why! I was working on a greek themed set before theros, got bored and have made it in someone's head :l I need to stop taking advice from my psychic powers.

Maybe they were created for that purpose, but if we (the audience, not the Vorthosi) react to them positively, it would be stupid for those 'walkers to never come back. And regardless of what other people say, WotC is generally not stupid.

They've apparently already been pushing for Tamiyo to make a return in one of the blocks they are/have working/worked on.

Ooh, that's good to hear. Do you have a link to your sources to share?