With Rel 3.5out it is possible to do some basic theory crafting on the roadmap topic for the girls themselves (though not for the game in general).
We know now the new tiers for the Resort guests (unique, main, regular and side characters)
General observation
The girls are now differentiated with clearly defined roles and futures.
Also it is possible for girls to be declared completed.
We should therefore expect to see more and more girls go complete.
Either as in done, or with a future new girl added to drive the content for an area ex. Natasha's for an ultra busty black.
Resort guests
Yvette and Amy are formalized as unique characters and will thus continue to spearhead content.
Given the rise of the specials, the lab and planned girls, the regulars are mostly stars belonging to the older era (though Ivy is likely to be prominent for some time while she is remade and catches up).
The side characters are formalized as being thus and given Delizia's transformation, it is likely Renee and Lacey (the other two non-functional resort guests) will also transform to staff.
Natasha will be completed and a black and busty will handle future content for that type of girl.
Specials
While it is not possible to accurately theory craft the special's tiers, it seems reasonable to proxy them nonetheless (Joy being an exception as she is pure story) based on a story/lab basis.
The lab girls and the lab itself are prominent, with large investments, so it seems reasonable to assume one or more of the lab girls will be a main of sorts and that all specials will continue to benefit in general.
Yumiko is the likely exception. She does not have a lab connection and is pure hired help. She looks like a staff prototype at this point (and would be likely to convert).
General Note.
This is
theorycrafting based on Rel 3.5 and the new tiers and design.
It is my opinion/guesses.