Evan Rachel Wood could return in Westworld season 4, but not necessarily as Dolores. HBO's Westworld took a radically different turn in its third season, moving the setting away from the wild west and into the futuristic real world, within which Dolores was running wild after her escape from Delos. Westworld left Dolores' intentions for humanity deliberately vague, and both Bernard and Maeve attempted to stop her deadly crusade. Ultimately, Dolores did not want to destroy mankind and steal Earth for her own kind. Instead, the former farm girl was seeking to destroy Rehoboam, the computer calculating humanity's every move, and while the revolution she sparked was certainly violent, it also succeeded in liberating humanity.
In a tragic sacrifice, Dolores gave her life for this cause. Desperate for the key to the Sublime, Serac hooks Dolores up to Rehoboam and searches her mind. He doesn't expect that Dolores will win over Maeve and use the last dregs of her mental energy to pass control of the entire system from Serac to Caleb, who represents all of humanity and makes the decision to switch off the spherical machine. Although many viewed her as a threat to humanity, Dolores forfeited her own life to give man control of its own destiny.
The concept of death is extremely loose in Westworld but both producers and cast members maintain that Dolores' death was the real deal and warn fans not to expect a miraculous resurrection. However, when asked by Variety if Evan Rachel Wood was leaving Westworld, Jonathan Nolan (co-showrunner) replied "I f**king hope not." Having been so adamant that Dolores' was gone for good, this answer hints at plans for Evan Rachel Wood to come back as a different character. The actress certainly wouldn't be the first cast member to play multiple roles in Westworld, after all.
If Evan Rachel Wood was to return to Westworld as a character other than Dolores, how would it work and who would she play? Westworld has always indulged in a sense of duality, whether that be between humans and hosts, WestWorld and the real world, or Arnold and Ford. In season 3, Dolores split her own personality into several other hosts, one being the guise of Charlotte Hale. By the finale, Hale had broken free from Dolores, developed a personality all of her own, and started making her own army of hosts. Continuing Westworld's love of mirroring, Hale could put part of herself into a host that looked like Dolores (the Evan Rachel Wood form). Perhaps Caleb becomes the leader of the human rebellion and Hale dresses up like Dolores to take advantage of Aaron Paul's character, who was clearly infatuated with his android friend. Evan Rachel Wood playing Charlotte Hale pretending to be Dolores - how very Westworld.
It's also important to remember that even though prime Dolores is dead, one of the offshoots she constructed is still in play. Aside from Hale, Martin and Musashi, Dolores also imprinted herself into the Lawrence host, and it's this version that makes contact with Bernard in Westworld's season 3 finale. Since he's carrying out Dolores' orders, Lawrence has clearly not gone rogue in the same way Charlotte Hale did, and the episode leaves this host still active in the real world. If Law-lores is still alive, he might try and reassemble Dolores using himself as the blueprint. Without the original article, however, this process is bound to go wrong, and the result could be Evan Rachel Wood playing an entirely new host, born out of Lawrence's attempt to Frankenstein Dolores back to life.
However Evan Rachel Wood returns to Westworld in season 4, it's vital that Dolores as the audience know her remains deceased. The character's season 3 sacrifice was incredibly meaningful and vital for the arcs of Caleb, Bernard and Maeve. Bringing Dolores back exactly how she was before risks undermining that.
Westworld season 4 is currently without a release date.