What is considered "mid game"?

[MonthOLDpickle]

What level should one start the expansion?

[Fenix]

Depends on difficulty level and your build.
15 is for normal, add 3-5 fro every level above Normal, less fro ranged, more for melee.

[harperfan7]

I started it at about 17.5th level on dominating; I'm doing fine.  The real thing to watch out for is the order you do things in; I suggest following the questline(s) rather than exploring, at least until you hit your stride (in the black sea, I mean).