Your points were good and then you kinda lost it at the end bringing up classes.
1.0 somewhat tried this when jobs first came out, they were advertised as only being necessary when doing full party endgame content and that classes were supposed to stay relevant when doing any other activity (low man / solo). This obviously didn't work, classes have no place really and are generally uninteresting in comparison to jobs.
If you want to change the need to keep raid focused balance then the way raids are designed need to be changed from the ground up.
Excessive reliance on DPS checks as measure of difficulty works for balancing content for the Duty Finder as people will eventually meet those checks via over-gearing but leaves no room for unique jobs / ability types that aren't focused around doing more damage. Where does a support fit if they only buff and don't actually contribute damage to the raid? The party might feel they're better served just getting another damage dealer instead to meet to DPS check.
The same goes for items like resist sets etc, the question becomes does it help kill the boss faster? if it doesn't then it becomes instantly worthless.
Change circular arena design and the timed DPS checks for raids, give us more fights with levers to pull (Leviathan EX) or adds to kill in addition to the bosses (Bismark EX) but don't tie them to an overall DPS check and you might start to see some variance in the types of jobs, abilities, and stats that we'll see.
And in regards to adding crazy cross class options to classes, why not just add them to jobs? as it is the cross class options for jobs are cookie cutter and boring really, I'm sure the argument will be made that players will find a "best set" of cross class actions but it'd be nice if i could pick and choose from more abilities tailored towards various fights.
There is no fundamental reason why jobs cant operate on the cross class system the same way classes can other than the fact that we just aren't allowed to, i would much rather see that changed before trying to make classes relevant in any way (even in solo content), sorry.