While I would put it in different terms, and would focus on different issues, I generally agree with the OP's sentiment. Stellaris needs feature fixes, not new content. Of course, without the new content there would be no budget for feature fixes, so bring on the DLC's.
Personally, I'd work on systems in this order:
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Economy (obviously in process)
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Writing - Stellaris desperately needs a lead writer to build its world. Yes, this is a sandbox game, but if anything that means they need to focus on the writing
even more. D&D is a sandbox game, but look at the attention to detail that goes into writing each monster, each spell, the history of Faerun. Stellaris needs the same, and it should be an ongoing process.
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Empire characterization and asymmetry - Empire types need to feel, play and be differently.
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Diplomacy
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Exploration - Exploration, with its discoveries and decisions, should change the course of your game, from
starting wars,
sparking alliances,
unleashing hell,
founding religions and beyond. Events right now have neither stakes nor consequences. Instead, they should shape the game as you play it. On a regular basis the decisions you make now should set the stage for the conflicts and confederations to come in 200 years.
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Leader characterization - When I put Sisko in charge of a space station, it should damn well matter. When I discover that I can't remove Palpatine as Governor of the Antaran Sector, it should be reason to panic. Stellaris wants to tell stories, but good science fiction is about people, not technology.