What determines the operating point of an op-amp?
The input voltages need to be setup differently. You have one side increase while the other side stays the same. That goes against the entire point of a differential op-amp. You need the input to have V- go down while V+ go up simultaneously. That's why you're getting the weird knee in the output.
To make the line ramp steeper, the fastest/easiest way to do that with this op amp is to increase the current flow through your current source. This will source more current in differential pair and the common-source amplifier stages because you've got that current being mirrored through the pmos's. If you can change the dimensions on the transistors (length and width of the channel region), then you have more knobs to play with.