Title says it basically. Would be great to have the option for cmd + drag in corner to have the window on top of any other window until it's moved again
Unfortunately this is not possible or would at least need System Integrity Protection to be disabled as it would require hacking into system code.
you can use MySIMBL plugin for that (winBuddy i guess) but as Andreas said - SIP must be turned off, at least for the instalation time.
Jup there is also "afloat" which I have used ages ago, but nowadays all of them require SIP to be deactivated ;-( (until 10.13 only temporary, I think on Mojave permanently - need to check that ).