A concept that I haven't seen much contemporary discussion of is effort. A jstor title search finds 18 entries, most notably an 1897 piece by Dewey. But there is very little recent material: There are a few interesting-looking pieces from the 70s (one on effort and desert, and two on effort and freedom of the will), and there is 2009 Business ethics piece on withholding job effort. The subject seems to have been largely neglected in recent times, though it was an active area of interest at the end of the 19th century (in addition to the Dewey piece, there is William James' 1880 Feeling of Effort.
So, if some graduate student needs a project in moral psychology or metaphysics, effort might be worth some attention. A dollop of history—thinking about conatus, say—wouldn't be amiss.
What made me think of the topic was struggling with wind in a canoe, and being struck by the idea that the feeling of effort or struggle, while unpleasant, does not seem to be classifiable as even prima facie evil.
Ability comes in degrees. I can easily lift a pound. With a moderate effort I can lift forty. And over that, effort significantly increases and ability peters out. There is some kind of a connection between ability and effort. Where ability is limited, at the edge of ability there seems to be a need for effort. So it may be that thinking about effort would help advance some of our understanding of ability, and hence of freedom of the will. But that's speculation. (On a connection with freedom, also see this and the responses in the above-mentioned jstor search.)
Here's an argument that has just occurred to me. The more effort of will, mind or muscle it would take for x to A, the less we blame x for failing to A. But the limiting case of increased need for effort is impossibility. So if Aing is impossible to x, for reasons of will, mind or muscle, then we will least, if at all, blame x for failing to A. This is a kind of principle of alternate possibilities. I am not that impressed with this argument. While the limiting case of increased need for effort is an impossibility of some sort, there may be sorts of impossibility that are not limiting cases of increased need for effort.