Not gonna happen. They tested the water with Scholar/Summoner (Plus they needed a second healer for ARR, because Paladin was the 'secondary healer' in 1.X) and there were balance issues, and some stuff can't be gotten rid of without issues (Like useless Physick on Summoner) and they weren't gonna be doing that again.
All jobs in the future will have its own job crystal and start at a boosted level, in which you need to be in the right expansion stuffs to have access to.
If people want to try out things from lower levels, they can easily go into lower level dungeons with squadron stuff/ect, or even just use a striking dummy for a while to practice the new stuff.
You just need to unlock the class and test out what it has to see if it feels like anything you wanna try out, and they have options to try out said things too.
And they're HIGHLY likely not gonna make the newest classes free for everyone to use. They need to keep newest 'Ohhhhhh shiny!!!!!' things behind some kind of pay wall to access (Buying the expansions), because those are times where they make huge boosts of money.
Also they will make both new classes start off at the same level as one another (lv30 for the 3.X classes as an example because they started with job crystals, Sage and reaper both started at the same boosts levels, same for every expac with new classes)
But these are things that 'You would like'. 99.99999999% chance they won't happen the way you want them to.