Apparently this is a hot take, but I don't think anything needs to change about BLU's design. It's niche, yes, but it is insanely good in the niche that it fills. They keep making us do old content to farm relic weapons, moogle tomestones, wondrous tails, etc. You can do these things on normal classes but it takes forever and it sucks a lot. With BLU, instead of being a slog of pressing the same rotation you always do but with less of your fun abilities, you always have all your abilities and you can tune the hotbar to the instance for maximum speed.
That last set of moogle tomestones? Farming CoB took something like 17 minutes per run on a normal class (I tested it with a friend - very small sample size and we wiped a couple of times), but on BLU it took 2 minutes with some minimal coordination due to the trash not being immune to Lv 5 Death.
The last xpac's relic grind? Fates. More Fates. More fates. Horrible on normal classes. Take BLU, yoink all the enemies toward you, freeze em, delete em. Even when ultravibration isn't up the freeze still gives you time to wail on a much larger group than you can realistically tank (without a tank + healer) and burn em down. When there were PF's for the raids, every BLU in that alliance reduced the time investment of the raid overall considerably.
In an ideal world, sure, I wouldn't mind if there was a separate unlimited job version, and I would be very happy if there was some way to play BLU in duty finder (required spells or whatever), but this is not an ideal world. Square Enix will not make the time investment to balance both sides, so if they are going to giveth unlimited pve, they will taketh away everything that makes BLU actually good and fun. I couldn't care less if they make more BLU specific content or contrive reasons to play BLU for unique gear/drops, as long as I can contrive ways into using it for the parts of the game that otherwise really, really suck, I am happy with BLU where it is.
That said, really need that level 80 update before the next relic grind. Super not looking forward to doing ShB content as an objectively weaker "balanced" class.