From a design perspective, simply tone down the hamfisted trope expression throughout dialogue and imagery... this would go a long way (similar to my opinion of Shivering Isles exp./Sheo... relying overmuch on random access humor to depict 'insanity', instead of actual disturbing imagery/scenarios). I understand ES writers from at least Morrowind through ESO have to cover a bunch of content with regards to lore and character expression, so I'm not going to say that the DB's execution was "laziness", but it's certainly something that can be addressed with any future appearances.
As for the lore implications, other posters have covered it well enough. If the TES IV DB portrayal is a result of some corprorate decision (e.g. the feared Destiny-style 'dumb it down this too nerdy'), I understand it from a business perspective that it 'works' for the average fantasy/game consumer.
However, I will take this topic as a soapbox for a broader point, I challenge them that they would be pleasantly surprised by consumer reaction if they took both these design and lore directions to the next level with future ES titles. Time and time again fantasy gamers have proven that they enjoy being intellectually treated over being patronised (with regards to writing). Again, the Destiny example: look at how many dedicate time to researching the 'abandoned', highly metaphysical/intricate lore direction that was hinted at before Activision told Bungie to knock off the nerd crap. I'm not saying jump into full c0da page 1 from starting hour of the game, but definitely explore all of these nuances and bring them to the forefront of plots instead of leaving them as nods towards the TESlore posters and lurkers. Stepping off the soapbox and bringing this back to DB, they could achieve this by defining Sithis and all as more than just 'muh cult object', what role Sithis has in the metaphysical arena of TES, and portraying members' personalities in a more grounded manner, acting as a reflection of what the DB really is rather is.