Also, what happens if the directory the user wants to work on has no attached RenPy(and py)? BANG! Exit 1
If the user is advanced enough to move the rpyc/rpa files outside of the game directory, then he's also advanced enough to know how to deal with this situation. Generally in this case he also don't use unren, but directly the standalone version of unrpa and/or unrpyc.
What would you do instead of the menu for all the choices?
A menu... but not in the batch file.
It's not that hard to make a script that works on both Python 2 and Python 3.
It's more difficult than you think. There's no backward compatibility between Python 3 and Python 2 ; it's in fact the reason why the branch 3.x was created. And __future__ is, alas, far to solve everything.
Anyway, having a Python 3 compatible script for unren is not the good approach.
There's a reason why PyTom will, once ready, maintain a 7.x (Python 2.x) and a 8.x (Python 3.x) branch ; the 7.x will end only when the 8.x will finally be totally stable.
He will surely not just convert the code of Ren'py, but also probably proceed to many refactoring. The migration being the opportunity for him to get ride of a lot of legacy code obsolete since years ; by example the buttons still understand the "clicked" attribute, that was replaced by "action" something like 8 years ago.
This would gave back its strength to Ren'py and make it ready again for a lot of improvements. This without its actual constraints. But this also include that both the RPA and RPYC "format" risk to change ; especially the RPYC one.
Therefore, until we see what will effectively be the 8.x, we have to assume that it will imply a change in the tools used by unren. Which would lead in the end, to a batch also testing if Ren'py have a "pythonlib2.7" folder, or a "pythonlib3.x" one, and launching the right "Python unren" according to the result of this test.
It will anyway be easier than a single script juggling between Python 2.7 and Python 3.x, and between the old and new formats of Ren'py files. Especially since the version for Ren'py with Python 2.7 will stop to need updates.