I was just thinking. Tiamat is the most powerful god now - is this intentional? If so, why?
I seem to recall in 4e lore at least, that Tharizdun is the most powerful and that the gods had to coorperate to defeat him.
I don't think CR is a good mechanism to represent things like solo vs gang.
The only reasonable way to make one monster (even gods are monsters in D&D) capable of taking on more than a handful of monsters (Not to speak of "all gods") is by absolutes.
Such as a driven rank saying "you're immune to the attacks of every lesser being".
Otherwise Tharizdun would have to be absurd CR. Like 20+ CR higher. And not even a CR 20 hero can take on a world full of CR 1's; that's simply not how 5E math works.
It just so happens that I use a Sumerian "Mother of Monsters" version of Tiamat, so I'm cool with this.
I've been leaning that way too. This version is sort-og half way between god & primordial. I was planning on making her a primordial to get back to her mythological roots, but I can't get past the nostalgia of reading through my 1e MM and being mesmorized by her entry. So I came up with something that is more than a god, but less than a primordial.
I never liked her as a god (Bahamut neither). I always saw her as the Ur-dragon - as powerful as a (lesser) god, but still "just a dragon". With the introduction of the primodials, I think that is more fitting (albeit less powerful probably).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.