Loved my G5. A huge beast of a machine but a lovely design nonetheless. Would be great to revisit the beautiful case and build something more modern in it sometime.
I am still happily using a stock(ish) Xeon-based 'cheesegrater' tower at home, but if you wanted to just reuse an old (PPC or Intel) Mac Pro case to build a new system in, there are some ATX adapter kits available[1].
1: https://www.thelaserhive.com/product/mac-pro-atx-kit-with-ps...