Quick trivia for you. What, other than being fictional deities in Dungeons & Dragons, do Lolth, Tiamat, Vecna, Bahamut, and Asmodeus have in common?
Answer: None of them were deities when they were first introduced.
As D&D has progressed through the various editions, there’s been an ongoing tendency to transform important figures of the mythology/background into gods. And honestly, I think it’s an unfortunate tendency.
There are lots of gods in the various settings already. Many of them are interesting. We don’t need more of them–certainly not at the expense of existing characters. "Vecna, Most Dangerous Lich in History or Legend Who May or May Not Still Exist in Some Form" is, in my mind, far more interesting than "Vecna, Just Another Evil God." Tiamat is far more interesting to me as the most bad-ass pinnacle of evil dragons, an ancient "mother of monsters" type akin to Typhon or Echidna from Greek myth. Lolth is much more interesting as a demon, with the drow a demon-worshiping race, than as a god. Etc.
It’s quite possible to have legendary creatures and villains who have just as much impact as the gods do on game history without insisting that they all be divine. I’d like to see not only new characters created in that vein, but I’d also like to see some or all of the aforementioned reverted. Variety–both in terms of backstory and in terms of the nature of these semi-mythical figures–is far more interesting, and makes for far story and adventure opportunities than Yet Another Evil Deity on the list.