I’m Very Bad at head-casting; I don’t have a deep vocabulary when it comes to actors, which is something this kind of fandom fun usually relies on. That’s a bit of a weasel answer. I’d like to see someone actually slavic cast in the role. But I’ve enjoyed Scarlett Johansson and Bridget Reagan in their roles— I like Natalie Dormer too and would be excited if she was cast. Lbr, though, I’d be excited if Yelena was cast at all.
The Soviet / post-Soviet contrast between Natasha and Yelena is probably not feasible in the MCU, even though the timeline in Captain America: Winter Soldier is a little warped. (Born in 1984, but working for the KGB?) That theme is probably less obvious and visceral now as it was in 1999. So on that level, the generation gap between Natasha and Yelena is less relevant.
Still, the goal for Yelena, in the beginning, was surpassing Natasha. She beat Natasha’s old Red Room test scores and was awarded the codename Black Widow after Natasha defected. Another theme of their early encounters was learned experience versus lived experience, something that Yelena can’t really embody if they were peers. You could still do a rivalry, still have Yelena determined to beat Natasha, but if Natasha’s always beaten Yelena at everything, if Yelena only got her legacy as a second choice, the stakes change. You have to introduce something else to make Yelena a heavier threat.
Maybe the thread could be that Natasha has spent so much time fighting robots, aliens, and Captain America that her actual spy skills have gotten soft, something Yelena takes advantage of. That Yelena would be a knife-sharp for a Natasha that is still figuring out how much of a hero she can be.