Would anything given to joker now really make him op? And all of my suggestions(may be altered slightly) are viable improvements...I only agree on his f21 being cancel able.
Gas can improvement is almost useless, even if the gas can was 17f it would make almost no difference, especially in the matchups in which he struggles.
Variations would only be good if the gas can was 17f and you could throw it fullscreen, that would have some little use as long as you could MB it for a fullscreen gunshot combo out of which you would instead dash in.
F21 would be useless to special cancel because we don't know the cancel speed.
MB flower is extremely meh, Joker doesn't want + as much as he wants to open up his opponent, you could do something with this in the corner but Joker's teeth take over 30 frames to come uot.
The bazooka would be straight out broken vs zoners.
B1 comboing into gas can is also meh because it encourages gimmicky play of that sort, it's a trap for the Joker player more so than the opponent.
Meanwhile, dashes allow him to get in on Zod, Sinestro, Aquaman, Zatanna and others and gunshot allows him to break Grundy's, Bane's and Lex's armour. All these changes make his bad matchups ( except Lex which is now 5-5 ) way easier and he is now a tournament viable character.