Ford dying wouldn't allow his essence to transfer to a new actor. A young Indy (it's not like we haven't see 3 other actors play him) starting as a grad student under Ravenwood, meeting a teen Marion, juggling his desire for fortune and glory with academic research and developing a moral code, THAT'S what audences want.I am not sure if changing the actor in 90s would do a trick. Indiana Jones was just too attached to HF identity. Personally I think Indiana Jones won't work as a different actor as long as HF is alive.
The only think you'd have to handwave away is the notion that Raiders and the Ark (or really Temple with the Sankara stones) is the FIRST time Indy has seen something faith based/spiritual/supernatural. But X-files showed us how to do it, Indy is a skeptic, there is an explanation for everything or the magic goes unwitnessed, not every story has to have gods in it, and the more believer types are his rotating group of companions.
But anyway, to relate this back to RDJ as Doom, I don't think RDJ would come back just to be a voice, he doesn't really have such a unique delivery style that he's an obvious choice, so they are paying him to BE Doom, which means no mask. RDJ has been a snarky actor practically his whole career, it's what he does best, and I gotta think that's where he'll take Doom. It'll be Stark 2.0 within 2 films, guaranteed.