I think since we know Steve Rogers is in this show but there has been no discussion of Chris Evans returning, and Evans has said he's done with the role, that they are almost certainly getting someone to do an Evans impression for Steve, so if they're willing to do that I reckon there's a very good chance they'll get someone to do a Downey impression for Tony.

I wouldn't mind having all actors recast for What If...?. Its voice-acting we are talking about... I don't think we will need a real Chadwick Boseman, for example, to make the impression of Black Panther.

He's probably going to return.

Chadwick Boseman is confirmed for What If...?

What if Spider-Man was iron-man

Ummm.... Good idea butttt I heard that RDJ is returning.

Idk if it's just a rumour.

Maybe RDJ just ruled out future films, but said nothing about voice work for a streaming service show.

Its a too expensive actor, when you can have other actors that don't need too much money to be paid.

I don’t think money is an issue for Disney. If they want RDJ to come in to record some lines for an episode, they will pay him. I’m sure they will make it work. I just don’t see Feige saying that was a reason why he wasn’t brought back, if that ever becomes the case.