Perhaps a Transcaucasian SSR? It was an actual thing and broke into the Georgian, Armenian, and Azerbaijani SSRs sometime after Stalin became leader. Another possible SSR would be a Greater Finnish SSR, in the possibility that the Reds won the Finnish Civil War or the Soviets got Finland after World War 2 and gave them the Karelo-Finnish SSR, a Manchurian SSR if the Soviets annexed the area instead of giving it to the Chinese Communists, or a Polish SSR if the Soviets completely annexed Poland in WW2 or won the Polish-Soviet War.
Other than that, the only possible options I see are a Karakalpak, Crimean, and a "Prussian" (although the term would likely not be used) SSR in the territory of Karakalpakstan, Crimea, and the Kaliningrad Oblast. This would likely end up being problems for the latter two after the Soviet collapse, as the regions would be majority Russian.