Can I wire a 120v 20A outlet with #8 wire
There is really nothing wrong with having a bigger wire. Not knowing how long your future is.  Would put in a over sized conduit in emt to handle ,future wire.  1 inch emt. pull in your 12 wire. when you do change it out .Use wires to pull in new ones. Codes may change by the time you upgrade.And wires may not meet new code and be waste of money spent.  Or pull your 8 wire add pigtails fine to .