It's a little unclear what your goals are for private practice. Is it going to be cash only or insurance-based? If the latter, are you going to take Medicare? Because that will determine what types of patients will be coming to you (e.g., you probably won't get a lot of private pay patients with schizophrenia).
Are you trying to market to the most people available with these certifications? If so, then you can market whatever the latest popular fad is in psychiatry. At this time, it would probably be intranasal esketamine, psychedelic assisted psychotherapy, TMS.
Are you trying to get as many clinical experiences as possible to treat a broad range of clients? Then take inventory on what you haven't had much experience with yet, particularly with diagnosis and pharmacology. Sleep medicine including the sleep meds outside of benzos/z-drugs, eating disorders, dementia, getting comfortable using clozapine and managing both hematological and non-hematological side effects of it, using long-acting injectables including non-antipsychotics such as vivitrol and sublocade, using MAT for alcohol/tobacco/opiates minus methadone, treating eating disorders,
Do you want to set up a therapy practice too? Then get extra training that you might not have already had in residency, such as trauma-informed therapy (CPT, PE, EMDR), DBT, ACT, couples therapy (Gottman, EFT), hypnosis for dissociative and somatic symptom and related disorders, MBCT, biofeedback, ERP for OCD, CBIT for tic disorders.
Are you interested in forensic work? This can be a good supplement to your future private practice that you can get experience in during an elective in residency. See if there are any faculty doing IME/forensic work and ask if you can help them with a case.
Regardless, I agree with sushirolls that getting business experience when setting up a practice is something that residency programs are woefully terrible at. Reach out to your APA district branch to see if there are private practice psychiatrists that are willing to chat with you about how they set up a practice. Meet with those who are a few years in, 10-15 years in, and nearing the end of their career in private practice. Read the APA Starting a Practice reference guide. Watch videos online about how to set up a private psychiatry practice. There are also some good articles you can search through PubMed. Meet with a CPA who works with doctors who've set up private practices. Learn the pros and cons of forming an LLC vs other business entity structures. Collect referral resources in your area where you can send people to. If you're going to take insurance then start getting credentialed with them and start negotiating rates. Consider joining one or two group practices part time while building up your private practice so you can learn the lessons from those structures. In terms of what you can make into an elective, see if you can be part of a group practice that your university or hospital is affiliated with for an elective.