If R2 had received a transmission at the end of the movie, I’d expect that would be from Luke detailing the location of the Last Jedi Temple and his route. In other words, R2 would have received the small piece of the map that BB8 has. That would mean that BB8 would have had the larger map, which would have showed the entire galaxy except for the piece that Luke would have still had to travel when he gave the map to Lor San Tekka.

Of course, that would mean that nobody would have been able to assemble a full map until the transmission, making the scramble by the Resistance and the First Order rather silly in retrospect.