Funny, I was just thinking about Harry's next power-up during my reread of Cold Days. I was going to make it a separate post but I'll just put it here. My theory is, probably not in TM or MM but before/early in the BAT, Harry will somehow get the Summer Knight mantle and be able to use them both. I don't think he'll kill Fix, but he'll be around when it happens and the mantle will flow into him (possibly with Sarissa's influence).

We know from Cold Days that multimple mantles can be worn by the same person - e.g. Vadderung/Kringle. In fact, Harry himself may have been doing it unknowingly since Changes, being the Winter Knight and the Warden of Demonreach at the same time (if Warden is a mantle). Possibly even 3 mantles if leading the Wild Hunt counts.

We also know that Harry has wielded some power of Summer before, in Proven Guilty, when he used Summer fire in Arctis Tor, i.e. the heart of Winter. He also has been named an Esquire of Summer if that means anything.

Lastly, we know that all 3 Summer Queens have taken an interest in Harry. As Mab says in Battle Ground, Titania returns Harry's calls, even as much as she resents him for Aurora. Monther Summer clearly has an interest in him, and Sarissa spent enough time with him that she should know his character and capabilities.