Blizzard bothered with Outland and Northrend because they contained characters and storylines that people cared about.
There's nothing worthwhile in Random Planet #22 that was invaded by General DoomBob in Legion.
I guarantee you that Blizzard's resources are better spent simply revamping Outland, like Midnight will do with Quel'Thalas and TLT with Northrend.
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An expansion centred around Denathrius, the Nathrezim, and the Legion's remnants can simply take place in a Revamped Outland.
1) Outland was always a staging ground for the Legion's troops in their invasion of Azeroth, all the way back to WC3 (it's implied that Archimonde was waiting in Outland for Kel'thuzad to summon him);
2) People actually care about Outland: