I really hope they ensure unsync being of very nearly the same. I find it is a very enjoyable feature, and feel it would be unfortunate to lose. Echo might help with that certainly!
Or, lol, they keep ideas like Basic Instinct up with blue mage such that it functionally replaces that with equal to greater power (equal to unsync your main job). A few thoughts to that might be Basic Instinct can trait upgrade later to give you all three mimic states at once (allow blue the sustain and tank it needs to solo some content easier, as well as giving solo blues a full kit building experience). Rather than hyper boosting damage too much more then is may consider adding a loot buff when you 'sync' with blue (new token more treasure drops, etc- so it may take a little bit longer than classically an unsync max job would but you get a bit more gameplay and the reward is boosted to reward that extra time, double benefit). Of course still able to complete content that an unsync could have done and perhaps more since blue has some wacky spells. Like instinct you might see Angel Whisper which has classically been auto-raise would do that when solo, keeping the cooldown naturally (and being the same as it is in parties, although I think when you are healer mimic it should heal a bit to feel more classic). Or a new spell called molt which causes you to shed a clone that takes any marker on you and temporally your enmity, when solo / basic instinct it might have charges or a better cool down and it can behave differently for party situations.
With an aside concept of a new token blue mages get for doing content synced (including carnival) which might be used to purchase the usual cool things, other currencies, and a special consumable item (incense) that massively boosts your damage reduction and increase some stats (either at great token cost or cost to all drops from the content) so if you can't get a group for those raid spells you could still at least learn the spell with a little personal investment / cost.
Then blue would be that job you used to go back to old content, and you could have a decent kit since with the instincts and incense you wouldn't be out of luck if you missed the hot patch period (incense to be used on those much harder contents- costly enough in some way that PF isn't disregarded but still an option when PF is dead, and noting instinct already helps a lot on some more normal ones). If they did stuff like that and refresh their 1 - 50 spells with the quality of designs that are in 60 - 70 release spells (they're consistently more interesting, cleverly designed- like ultravibration is OP and relates to any deep freeze / petrify skill which opens future potentials, just works on what you expect not bad % spam, and has a cooldown so it doesn't fall to 1k needle spam), I'll go bury my blue salt somewhere deep lol ('advanced' job would still be neat but eh, at least I could use it and progress it even if I had to spend tokens or something to bolster effort).