Those are all dandy locations, which I hope they eventually do give this level of attention! (Personally, I would like to see Hestavar more than Sigil. Reading about it made me want to see a CG movie set there!)Don't remember a single thing about Union, never read the details. But I'm a bit pissed about Sigil: they already detailed it in the Manual of the Planes, why again? I would rather have two locations, one in Feywild and one in Shadowfell, or maybe some Astral Dominion.
Speaking of them, another option for DMG3 could easily be Hestavar.
Aww and what about Arvandor?? It could make a veeery good Paragon and/or Epic starting point. It's a Feywild into the Feywild (well, out of, but connected), and it screams out for high fantasy things, like FR Evermeet or the classic Avalon.
If you have a look at the Sigil description in the MotP, and then compare it to the Fallcrest description in the DMG, you'll realize there's no contest: The description in MotP is a mere overview, a travel brochure for Sigil, compared to the whole chapter the tiny town of Fallcrest got in DMG. DMG2 is promising to bring that level of detail to the 4E version of D&D's most famous planar city.