Defenders of the Ancient.. it's a MOBA, and the heroes do not work well with the style of Epic Seven. They are 3D characters, and depending on the version (Warcraft 3, Starcraft, Valve) has different looks, styling and to certain degree Lore. It has a big roster of characters, and there is no real story - no main protagonist, nothing, just characters who have some background and some rivalry, but it's like having one character from each game out there in one single world. Too dysfunctional in story telling.
That's why. Epic Seven is story driven and animation driven, and DotA has nothing of both.
Guilty Gear is story driven and animation driven and on top of it has very similar characters in style and combat.
Well, I can't agree with guys who commented above, art-wise and gameplay-wise it's totally doable, however it's quite impossible for valve to do collab with some unrelated gacha game. Until they maybe develop their own one lol.