That won't do, have players start from level 40 still no one can save them to get decimated by current players if they were to engage in fights. Neither you can't have them jump to level 60, and even levels don't bring activity since it's not like current players are much activite despite their higher potency
If something interesting were to introduced, something that can attract people, more than just levels, then it might work but as I said as long as One Piece section, in general, not active RP won't be either