I was watching a youtube video and came across this.
This is not my theory, It's by a Youtuber named 'SuperC4000 - Das Leben ist auch nur ein Spiel'

One Piece is an ancient plan to unite all the seas into one ocean. It was created by the Ancient Kingdom 900 years ago and is detailed on the Poneglyphs; the 'Ancient Weapons' are means for carrying out this plan; i.e, to destroy Reverse Mountain and Mariejois; the two points where all 4 seas meet. (Noah was built by king of the Ancient Kingdom, Joy Boy, to evacuate Fish Man Island so the FishMen would not be destroyed along with Mariejois). 900 years ago twenty kingdoms waged war against the Ancient Kingdom in a revolt against this plan, and the Ancient Kingdom lost. The Twenty Kingdoms united to form 'The World Government' and have ruled ever since - the plan to create the legendary sea 'One Piece' or 'All Blue' as other rumours would call it, were erased from history, and hence the 'Void Century' now exists. Gol D. Roger learnt this truth from the poneglyphs, and sought to carry out the inherited will of the Ancient Kingdom, or 'The Will of D'. However, he caught a debilitating disease and could not complete the plan. To pass on his will, he started 'The Golden Era of Pirates' in the hope that a worthy successor would take his place and fulfill the will of the Ancient Kingdom to unite the seas and in doing so, defeat the World Government.One piece will also create all blue (uniting all seas; north south east west blues) but breaking of the reverse mountain consequently also results in demolition of fishmen island so luffy will get one piece but is also seen as destroyer of fishmen island (as predicted by Sharley.