Kang is more of a Young Avengers villain.
I love Dr. Doom (in case the username didn’t make it obvious. Apparently not as when I made it I never even consider the Doom video games which I’ve never played) but I never bought him as an Avengers villain personally. The F4 can beat him so I don’t buy heavy hitters like Thor, Hulk, Strange, and Carol should have THAT much trouble dealing with him (and especially if the F4 are right there with them helping). Even with an army of Doombots and such at his side. Sure, they could do some “he absorbs another villain’s power” shenanigans but I dunno, I feel it is unlikely IMHO. He could utilize time travel to his advantage but I doubt the next Avengers film will utilize time travel given the previous one was mostly that.
Hell, in terms of the IPs that the MCU can now use, Apocalypse from X-men would fit a far bigger threat than Dr. Doom. But again, I feel they’d keep him to fighting the X-men only.
To be absolutely clear, not hating on Doom at all, and by god I wish Mads Mikkelsen could play him as he felt wasted as what’shisname from Dr. Strange 1. I didn’t think of it myself, when I was watching the Hannibal show with my friend, he suggested the casting and it blew my mind. But regardless, just because I love the character doesn’t mean I always think they’re best suited as a threat of any scale. I love Lex Luther but obviously Darkseid and his forces will always be a more suitable opponent for the Justice League.
On the other hand, Kang is essentially unkillable and could possess a way more massive army with ridiculous weaponry and ships from whatever verse the next one comes from, and then the next one, and the next one, and so on. None of them might be that powerful by themselves but they can essentially be a never ending force to just wear down the Avengers. And obviously this would utilize the multiverse and it feels Strange should have a major role in fighting Kang since he has had plenty of multiverse experiences already and also Scott and Hope since they will have presumably fought one version of Kang already. If Kang’s final defeat didn’t involve them at all that would just feel weird to me.
Plus, after Ant-man 3 Kang will have had two major appearances versus Doom not appearing at all yet. It just feels the former is being set up more, and of course the use of the multiverse.
Besides, I feel a young Avengers film is still far off. We only have Spidey, Kate Bishop, and Ms. Marvel so far who feel they would fit that team. Oh, and presumably Ironheart will air next year but that’s only four members. But an Avengers film, we still have Thor, Hulk, Hawkeye, Sam/Cap, Bucky, Spidey (no reason he can’t be on both), Dr. Strange, Ant-man, Wasp, Carol, Rhodey, Wong and potentially Shuri, Ms. Marvel (see Spidey) and Shang-Chi (maybe Moon Knight though I feel he works better alone and I’ve heard the comics follow the same idea).