Until such a time as an adult Lex is cast, we’re free to use our imaginations that Bruce Willis agrees to someday guest star on a CW show.
Stranger things have happened, and Supergirl has to bring in an adult version of Lex at some point, right?
The show couldn’t have introduced a young Lex and revealed the existence of adult Lex’s weapons caches if there isn’t the intention to eventually introduce him in all his bad guy glory.
Of course, the odds of a big name movie star like Bruce Willis signing on to appear on Supergirl even for a single episode aren’t too great, but it’s fun to imagine.