The Thinker’s Ultimate Plan Theory.
Ok ready? This is a bit crazy so bear with me. So if you read the synopsis for episode 4x09 which is right after the crossover we see that Barry will get kidnapped by Devoe (and something happened to to Caitlin too but who cares right now)
The question hit me, why would this villain want to kidnap the Flash? What is his goal? But after seeing tonight’s episode the answer hit me.
Devoe’s body is deteriorating, his mind slowly killing his body. But what if he had Barry’s rapid regeneration? His mind wouldn’t be able to destroy his body fast enough and he could live on.
That’s what I think he wants, the body of the fastest man alive. Inserting his mind into Barry, taking over him. And eventually Barry would be trapped in the back of his own mind unable to get out.
So when Team Flash rescues Barry at the end of episode 9 from then on they will start to realize that something is off with him. Sometimes Barry is in control and at other times Devoe is, and in those moments Barry finds himself blacking out for long periods of time and has no idea what’s happening. Devoe will continue growing stronger until Barry is unable to surface and he’s merely a puppet and the Thinker is holding the strings.
With the Fastest Mind Alive combined with the Fastest Man Devoe will be unstoppable.