I think a "comeback" could be as big as SHE wants it to be. It all depends on her finding her passion again. This conservatorship has killed her passion. Getting out of her situation doesn't necessarily mean that she will get her passion back but, at the same time, it is the only way. She has literal years of material she could write about and with the right song writers/producers (of HER choice) she could easily make an incredible album that will be a hit with fans and the GP. The fans would eat it up just because it would be Britney on her own terms, the SJW's would eat it up because of her getting justice. Casual members of the GP would be interested because of those 2 combined, as well as we know how much better Britney performs when she is feeling the material she is performing to. Having her passion back could introduce her to the newer generation as the pop titan that our generation knows her as.
Having a full sit down interview with someone she is comfortable with (who also in great at what they do being an interviewer) could do wonders for her public image. Unpopular opinion maybe.....but i think Kelly Clarkson would be great. She has such a positive energy, has also fought behind the scenes with her label/management. Recently went through a divorce where she has to pay alimony just like Britney. She would know as someone who has been through similar situations (given not to the same extremes or extent) what questions and topics would be kosher to speak about, and even the hard hitting question would be said in a positive manner.
Add in the fact the she could be around her sons whenever they want to be around without that douche bag interfering would automatically put her in a better space as that is all she has really wanted is a happy life with her kids. Her kids are the newer generation let's keep in mind! They know what is cool with their generation and could help her with that aspect. Not as managers or anything of the sort, but they definitely would know what she could do to be relevant with their age group.
Would the album become as big as BOMT/OOPS/Britney/ITZ? No, absolutely not, and I hope no real fans would expect that. Could it be as big as Circus? Yes, I believe so. Could it be bigger than BO, FF, BJ, Glory? Absolutely! Not to mention that having a decent/big era in the streaming era would boost her streams for all of her previous work and keep the buzz on her name relevant until her next project. The album may not move as many sales equivalents for the "comeback album" as the Circus album moved during its campaign but, with the added sales equivalents from the boost of streaming of her old projects, it could move the same if not more when all is said and done.