Wouldn't happen, it's worth reading Stalin on the national question to understand this. It'd have to be someone else, and a good chunk of the party leadership would have to be different, in essence, not the Bolsheviks.
Actually, Stalin originally wanted the RSFSR to annex all the (nominally independent in the early 1920's) Soviet Republics (Ukrainian, Belorussian, and Transcaucasian) and make them Autonomous Republics of the RSFSR, with the same status as, say, the Bashkir or Yakut ASSR's. Lenin however feared that making all the other peoples part of an explicitly "Russian" state would exacerbate national discontent, and insisted on the creation of a new entity, the USSR, which would embrace all the Soviet republics, including the RSFSR. (One reason he favored this is that he still dreamed of other European nations joining the USSR after a revolution.) Stalin had never been much impressed by Lenin's distinction between "soviet" and "autonomous" republics. "In your theses," he wrote Lenin in 1920, "you draw a distinction between Bashkir and Ukrainian types of federal union, but in fact there is no such difference, or it is so small as to equal zero."
https://books.google.com/books?id=smDy35onbtAC&pg=PA270 But ultimately he went along with Lenin, and the USSR was created, though the status of the RSFSR within it always remained something of a problem. (Almost to the end of the USSR, the RSFSR lacked some of the institutions the other Union Republics had, like its own Communist Party, own Academy of Sciences, etc.
So an RSFSR *instead of* a USSR seems a real possibility. What the initial post proposed, however, if I read it correctly, was something rather different: a USSR which would include Kazakh, Georgian, etc. SSR's but in which no separate Ukrainian or Belorussian national existence would be recognized, and in which Ukraine and Belorussia would become part of the RSFSR. This would be very unlikely. Bolsheviks, like other Russian Socialists, had always objected to the Tsarist attempts to suppress the Ukrainian and Belorussian languages, and ridiculed the Right's argument that Ukrainian and Belorussian were merely dialects of Russian that had been corrupted by Polish influence. Indeed, in the 1920's the Bolsheviks actually went out of their way to encourage national consciousness among the Belorussians (among whom it had been very weak--much weaker than among Ukrainians). There might or might not be a USSR; but if there was one, a Ukrainian (and probably a Belorussian) SSR would almost certainly be part of it.
There is also another, practical reason to have Ukrainian and Belorussian SSR's: many Belorussians and Ukrainians lived in Poland, and the existence of "their own" SSR's would be a magnet to induce them to seek to separate "West Ukraine" and "West Belorussia" from Poland and join the USSR. (The same consideration goes to Ukrainians who lived in Romania or Czechoslovakia. For that matter, the Ukrainian SSR exercised a considerable attraction to the Ukrainian diaspora as far away as Canada
https://en.wikipedia.org/wiki/Association_of_United_Ukrainian_Canadians though obviously this appeal waned after news of the 1933 famine became widespread.)