The best way would be for there never to have been a Belorussian SSR at all. To quote an old soc.history.what-if post of mine:
In 1917 there were few peoples in the Russian Empire with so little national consciousness as the Belorussians. In the Constituent Assembly elections, the only Belorussian nationalist party--the Belorussian Socialist Hromada--got 0.3 percent of the vote in the Minsk district, compared to 63.1% for the Bolsheviks, 19.8% for the SRs and 1.7% for the Mensheviks and Bundists. (In the city of Minsk the Hromada polled only 161 votes out of 35,651 cast. All these figures are from Richard Pipes, *The Creation of the Soviet Union*.) Contrast this with Ukraine, where the Ukrainian Socialist Revolutionary and Ukrainian Social Democratic parties got the great majority of Ukrainian, especially rural Ukrainian, votes in the election. To be sure, the Ukrainian SRs and SDs did not at this point advocate outright independence (neither did the Hromada) but they were unmistakably nationalist as well as socialist parties. No such party could attract mass support in Belorussia at that time.
Despite the evident weakness of nationalism in Belorussia. the Hromada convened in Minsk in December 1917 a Belorussian National Congress which proclaimed the independence of Belorussia. (The Congress contained a great many Russian anti-Bolsheviks who were no doubt motivated not by Belorussian nationalism but by a desire to separate themselves from the Bolshevik regime.) However, this new "republic" was very short-lived; by the end of 1917 the Bolsheviks had taken over Minsk and other major cities, and by late February the Bolsheviks gave way to the Germans. The German-sponsored Belorussian National Republic (BNR) could do little but issue proclamations, the German army having the real power. In any event, in December 1918, when the Red Army reoccupied Belorussia (after the Germans evacuated their troops, and the officials of the BNR departed with them), the Bolsheviks decided to create a Belorussian Soviet Republic. This was by no means an uncontroversial decision; some members of the Bolshevik party's Northwestern Regional Committee argued that the Belorussians were not a nation, and that it was wrong to artificially foster nationalism. But they were overruled, and the Committee was ordered to change its name to the Communist Party (Bolshevik) of Belorussia. In February 1919 the Belorussian Soviet Republic gave way to the stillborn Lithuanian-Belorussian Soviet Republic--Litbel--which was soon occupied by Polish troops...Ultimately Belorussia was divided between Soviet Russia--which re-established the Belorussian Soviet Republic which was to become one of the founding republics of the USSR--and Poland.
So suppose the Soviet regime had listened to the opponents of a Belorussian Soviet Republic, decided that the Belorussians were not a nation and that Belorussian was a dialect of Russian and simply incorporated Belorussia into the RSFSR? (At the same time I am assuming that they do *not* do the same thing with Ukraine, due to the greater influence of nationalism there. It may be inconsistent to say that Ukrainian is a language and the Ukrainians a nation whereas Belorussian isn't a language and the Belorussians aren't a nation, but the Bolsheviks were capable of worse inconsistencies.)
One problem is that, as I noted, the Soviets did not control all of Belorussia: from 1921 to 1939 western Belorussia belonged to Poland. However, I doubt that the Poles could successfully make Belorussian nationalism a successful anti-Soviet propaganda weapon (despite Pilsudski's advocacy of federalism in the early post-World War I period) in view of their treatment of their own Belorussians. (Of course I'm not saying that the Poles killed Belorussians on anything remotely like the scale the Soviets would in the 1930s; but neither did they show much sympathy for the Belorussians' national aspirations, whereas the Soviets did at least pay lip service to such aspirations, and did in fact--at least in the 1920s--encourage the use of the Belorussian language.) Conversely, the Soviets could not make the appeals to Belorussian nationalism they did to the Belorussians of Poland in OTL between the world wars; but I am not sure that appeals to pro-*Russian* feelings among this group, combined with reminders of their economic grievances against the Poles, would be much less effective. In 1939 the USSR would "liberate" the western Ukrainians and the "west Russians" from the "yoke of the Polish landowners and capitalists." In 1941 "west Russia" would fall to the Germans, to be liberated a few years later. Obviously, there would be no UN seat for west Russia.
More recently, of course, the independence of Belarus (up to now I have called it Belorussia in this post, because that is what it was to most of the world until 1991) would not have been proclaimed in 1991, and the issue of its "union" with Russia would never have come about. Lukashenko, assuming he is not butterflied out of existence, would at most be an obscure local politician in western Russia...
https://groups.google.com/d/msg/soc.history.what-if/cVLy054UpwM/PbSz_XukAp0J
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To this I would only add that the problem with incorporating Belarus into Russia after 1991 is this: the politicians in Belarus who were most contemptuous of Belarussian nationalism were also not altogether happy with the politics of the Russian Federation (especially under Yeltsin); and in any event, their idea of "Russia" was not limited to the Russian Federation. Besides, like most politicians they wanted to preserve their own power--even vis-à-vis a nation they considered to be of the same nationality ("Russian" in a broad sense) as their own.