I think we have two options to find a future Raj's girlfriend, to create a new Character or to recycle an old character
If the producers decide for the first option there are some actresses they can choice depending the type of girl they want for example:
The Ill-humored Asian Girl: can be like a art martial master to the gang, and let Penny can be the first season's sweet girl again, The Actress can be : Julia Ling.
The Cute and sweet girl: well I think another sweet girl can let to Bernadette rougher, she can be a former Bernadetteâ€™s classmate from the catholic school The Actress can be: Amber Tamblyn
The Committed Girl: a girl into green peace, or every good or extreme cause, can takes the gang to strange parades or asking why they waste their time playing instead doing something for the planet The actress can be: Odette Annable
The Normal Girl: another normal Girl can help Penny to feel not lonely in the nerdish world, and always will be great to see the guys teaching geek stuff to someone The actress can be: Mekenna Melvin.
The Eccentric girl: forget the hotness we want comedy so a eccentric girl is something we need, a girl into yoga, and vegetarianism, someone can drive Sheldon crazy with strange theories The actress can be: Lauren Lapkus.
The Indian Girl: ok that is too easy but imagine an Indian girl, hot and beauty, that loves her culture perfect to be the daughter in law for the Koothropali Family, the problem: she is the daugther of Raj's father rival, we will have a Romeo and Juliet thing in Indian style The Actress can be : Rebecca Hazlewood
if they decide to recycle Character I think they I would like to see one of them
Erin Allin O'Reilly: As Cheryl again, we need someone else in cheek cake factory
Elizabeth Bogush: As Dr. Catherine Millstone ,her character can forgive Raj attitude in The Pirate solution, work with him and fall in love with him.
Danika Mckellar: for every reason you said before.
Courtney Ford: she can come back and fall in love with Raj, forgetting hes rage against Leonard.