first of all I gotta admit I didn‘t play the PTR and therefore got no first-hand experience my own. I just obtained my knowlegde by reading through some threads and newsarticles. Just stating that, in case I‘m utterly mistaken.
So what I‘m about to suggest is to connect the scaling of the season theme damage, with your actual Paragonlevel and nothing else. Therefore people who invest massive amounts of time get rewarded and being a mere casual won‘t suffice to hit the highest Greater Rifts. In regards of the leveling progress before the Paragonlevels I also got two possible suggestions. Before you hit level 70 you might either have a preset damage value or just adapt the Paragonlevel scaling system onto the normal leveling progress.
Of course one might call the whole season theme lame or boring, that‘s completely up to you to decide, whether you like it or not. But what I state as like being completely for sure, is the fact, that Blizzard is pretty unlikely to invent like an utterly new season theme. So we might have to work with what we got and try to help, making it the best it‘s gonna get.