Most of the dedicated viewers commented here are so busy with analyzing the scene psychologically, which cannot go anywhere but speculation and wished-to-be-desires. Their observation skills need to be seriously questioned. The fact is, only the man who went to take a leak could shot Tony without his knowledge. But as The Sopranos Series show, there MUST be at least two shooters for this kind of thing. Let us remember from the beginning: The first season, unsuccessful assassination against Tony by Junior; two afro-American shooters. At season four, Carmine Lupertazzi's last minute cancelled shooting; again two afro-American shooters arranged. Last season, Doc Santoro's shooting with Phil Leotardo's giving; three Italian-American shooters. These are some examples of major shootings against the top level and many more can be found following this pattern (for one example; Bacala shooting).
Another fact: in the series, the shooters are extremely professional, dedicated. They are more like robots; show no hesitation, no sneaking around. They go in, go directly to the target and shoot. So let us think: at the last scene of the so-called Tony Soprano shooting, could the man with the gray jacket be a shooter as we had been shown for years? Well, the answer to this is also a 'no' by the show itself. (And for the blank scene situation let us consider also this tiny point: Tony remembered the conversation about death with Bacala right after Bacala's killing, while the family was under serious attack. He realized that he could never wake up again. But he 'woke up' (as strongly emphasized); he just had survived the most critical day(s) of New York family strikes. Though, that particular conversation and remembering likely seems that were only to be for that highly stressful night.)
Thus the most reasonable explanation for the last scene with all that awkward tense closing with the blank scene as well said before; a moment short symbolism of the life of Tony Soprano; embracing the joy of family while watching his back constantly. It didn't ended but could be anytime, says the scene.