There's a way for Reverse-Flash to return for the Arrowverse after Legends of Tomorrow season 2.
In Crisis on Earth-X, Reverse-Flash returned to the status quo established in The Flash season 1 by countless versions of Harrison Wells to merge into one being: Nash Wells. Reverse-Flash, not content to be trapped in Nash's mind, tried to take over his body. In The Flash season 6, episode 15, appropriately titled "The Exorcism of Nash Wells", Team Flash managed to "exorcise" Thawne from Nash's mind.
This doesn't mean that Thawne is dead, though. This is made plain during a conversation between Barry (Grant Gustin) and Cisco (Carlos Valdes) at the end of the episode. They both understand that even though Nash is free of Thawne, this may not be the last they see of the villain. It's mentioned that Thawne - who now has no body of his own - simply consists of a group of negative tachyons. That's why Cisco says Thawne will search for a new body to claim. If Cisco's theory is correct, then it stands to reason that when Reverse-Flash returns, there will be one key difference: Reverse-Flash will be played by a third actor.
To have a corporeal form again, Reverse-Flash may have to find someone weak enough that he can control. Once he can do that, he can be fully restored, but it's hard to imagine a storyline like this working without having a new actor in the role. Reverse-Flash's new host could really be anyone, even a female character. As for when this would occur, it seems likely that the series is done with Reverse-Flash for the remainder of the season. So, this new story may not unfold until The Flash season 7 or later.
Seeing a new actor play the part of the Scarlet Speedster's greatest enemy will be strange, considering that viewers have grown accustomed to seeing Cavanagh play the iconic villain. Alternatively, many fans would love to see Matt Letscher suit up as the Reverse-Flash for the first time since 2017, but having a third actor play Eobard Thawne could be a good move for The Flash. Even though both actors' performances have been met with a lot of praise, giving the role to someone else can avoid Reverse-Flash stories in the Arrowverse from becoming stale and allowing for a fresh new take on the character.