If Shanks had never startled Buggy on the Oro Jackson when they were apprentices, Buggy would never have eaten the Bara Bara no Mi, and would have been off somewhere under the sea finding treasure.
If Buggy was under the sea finding treasure, he never would have run into Nami and she would have stolen all the money from Orange Town and been long gone.
If she had been long gone, Luffy and Zoro wouldnt have met her.
If they didnt meet her, then they wouldnt need a reason to go to Cocoyashi Village, and he wouldnt have beaten Arlong and gotten a bounty.
They also wouldnt know how to get into the grand line, and would have crashed into the Red Line, and Luffy woulve drowned and died!! Sanji Zoro and Usopp wouldve disbanded,
The End, what ever you do, dont go back in time and stop Shanks from startling Buggy