Morkus from Orkus
There's no real power source niche at all these days. Arcane casters can heal. Divine one deal lots of damage. Arcane, divine and primal are just buzz words. Most of the uniqueness comes from class and subclass anyway.The problem with psionics in the past is, each power source − arcane, primal, and divine − had its own themes and niche. Since the themes that would properly go to psionics such as mind affecting enchantments and telekinetic force effects already went to the arcane source, psionics became inherently superfluous and redundant. Thus the implementation requires deep structural planning at the start of the game.
What is true in the past, might be less true in 2024. Its spell lists for each power source seem comfortable with spells that overlap several sources. Thus there is less niche protection, and more interest in simply picking whichever spells best match the themes of the list.