If we were to move Mix and bring in a winger, we could move TMac back to midfield as the first offensive option off the bench. That said, I still prefer bringing someone who can provide an upgrade to central defense.
Saw that Kekuta Manneh didn't play for a "coach's decision" for Vancouver last week. Maybe he's being moved. Imagine a front line of Manneh-Villa-Harrison. Yeah, chance of it happening is about 0%, but that would be filthy.
There are really 2 areas to go for in an upgrade: a better CB and a winger to start beside Villa and Harrison. If Mena-Brilliant is the desired combo (and I think that must have been the idea at the beginning of the season...you want the strong/smooth combo - i.e. Puyol-Pique) then you get that winger. It's also harder to integrate a new CB to the backline as chemistry is key with your starting CBs. But Hernandez should be #3 at best by the end of the transfer window.
You also need midfield depth, but that will happen if you get that winger and move TMac back as Gotham Gator mentions above.