Parmandur
Book-Friend, he/him
With the artbooks, they're still just previews.
And they need more playtesting. Seriously, go back and read those things. The Ahmonket one has a race that is just Aarakokra but with more stuff, some of which is really strong on it's own. The Khaladesh one has a feat that is literally just "Ritual Caster for people who think Ritual Caster is worthlessly underpowered, mildly flavored to look like devices."
The articles are very poorly balanced. Like, noticeable at the table in actual play.
Right, like I said, I think that the planes should get a joint book the size of Rising or at least XGtE, that delves into several worlds, and leaves room for advice on world building. I'd actually make this a book that encompasses some DnD worlds, as well, and put new player options and new rules that expand the game, like XGtE or Rising From The Last War.
Maybe call it The Gatewatch Guide to The Multiverse, or put a female NPC front and center for once and call it Chandra's Guide To The Planes, or something. I mean, Nyssa is cooler IMO, but Chandra is more popular.
I agree that the Planeshift articles are rather limited, really just the DM notes for James Wyatt's homebrew games played in the office, until Ravnica. However, it is interestign to note that the Magic team at WotC collected survey feedback for those documents when they came out, similar to the UA playtest feedback. So WotC has some player data from the crossover fans similar to what they did for Eberron for, say, Zendikar. So the basic player options that would be appropriate have already had public vetting and responses taken, which would lead to the natural next step of in-house testing if they wanted to use them.