I don't see Nel getting the circumstances to bring her back to fighting form.
Even in baby mode, Nel would still be an incredibly useful support utility. Why? Healing spit. With Orihime more focused on combat than in canon, a dedicated healer would be useful. If the Drifters had a way to fix her (canonically, sandal-hat made an artifact for her to do this), she'd also fix the hole in their squad composition. Even before then, she'd be willing to join the Drifters in a raid on Las Noches (outright canon!), where she can at least temporarily revert to adult mode under moments of severe distress (canonically, when Nnoitra was about to finish off a defeated Ichigo).
Actually, we totally should have recruited Nel ages ago. Given how lacking in trustworthy doctors Las Noches currently is after losing Esmerelda, Nel might have enjoyed Aizen's protection against Nnoitra finishing her off. Even this becomes unnecessary after Cirucci kills him off. In her adult personality, Nel's knight persona plays well with Cirucci's princess persona, and her fall from grace as the former Tercera could have played well as a mirror to Cirucci's loss of rank and recovery thereof. Interestingly, her Fraccions are numbers 41 and 42, making them some of the few Arrancar to have been transformed and present in Las Noches longer than Nemo. Adult Nel further fits the themes of the story by having a personality inclined to train just to gain skill -- as a weapon user in both base form and in Res, she'd see benefits even aside from standard ghost magic improvement.
That said, we never had an in-character reason to seek Nel out. Las Noches at large believes her dead rather than exiled. Ren or Spiderbro might have been able to clue us in, but Nemo wouldn't know to ask without Omicron volunteering the information somewhere in the text. And adding new characters to the story is increasingly unlikely -- the Vizards were already written out of the story due to character overload (this is definitely Kubo's fault). The time to have recruited Nel was long before we knew we'd want another Arrancar at just shy of Vasto power level who'd gladly fight Aizen for the greater good.