The prompt was Jund, so I made Jund. Literally.
The 'Soul of' cycle came to mind right away. A 6/6 for 6 with a keyword, a non-tapping activated ability, and the same ability again, usable from the graveyard but also exiling the card, and each part has to be thematic to the flavour and mechanical identity of the plane. But unlike with the existing cards, Jund isn't a plane, it's a fragment of a plane, so how would that work? Well, I didn't think about that right away, but the design did reflect that in the end.
So the first thing I did was look up all Jund-related cards just to remind myself what Jund as a place mechanically was about. And well, it's Devour. It's making loads of small stuff, then eating them for power. With that in mind, I decided the ability would have to make Saprolings (the token of choice in Jund apparently) and then as part of the same ability, make something with Devour. Not much tinkering with the devour creature, I ended up using the same token made by Dragon Broodmother. How many saprolings ended up being harder, until in my research I came across Spore Burst, which I riffed on here. So when you were just playing a Jund deck, you'd get 3 Saprolings at most, but in a full Alara 5-colour deck, you could get 5. Using a mechanic of a combined Alara to make the ability better? Yes please.
That's when it clicked about how I'd do the mechanics of a full Alara Shard 'Soul of 'cycle. Use domain on the abilities, but don't make them punishingly expensive to use for what you get if you're just using the three colours of the shard. Then finally, what the keyword ability should be popped up in my head. All 5 of the Souls would have Cascade, a keyword very specifically (in the Alara block, at least) representing a combining/combined Alara.
The art was an interesting hunt. I wanted something that felt like 'this is Jund'. So, what's in Jund? Plants. Dragons. Plant Dragon. Finding a suitable image that I could also track back to its artist took some time, but I eventually found one that looked like it grew right out of the plane and is gonna eat ya. Hurrah!
So that's 'Soul of Jund', and the ideas that I'd use if I decided to try completing the cycle. Which I might do, but it's not something I'm gonna do now.