what do you mean by improved output? if you mean increased gain, why not incerase resistance of load transistors or using wider input transistors! your input transistors are very smal (not wide), why? and is there a reason for the reason for the different output DC voltage or not? check these. also, why do you want it to operate in weak inv? Is it only lower power cons? if so, and you have considered the resulting lower bandwidth, higher missmatch and ...., then for example for the input stage you can do so by lowering the tail current i.e. lower gate voltage for M5 (make sure that this transistor has a high enough VDS to be in saturation) , meaning higher source voltage for M0 and M3. Also, are you using a tripple well process? if not, dont connect the bulks fo the NMOS devices to their sources. Connect bulks of all nmoses to ground.