OpenTTD way hopefully 
Currently we don’t have a proper replacement pipeline even in place, some bits here and there. And it is not a priority. It is a great deal of work for a complete assets replacement. And the new assets would not use the same techniques (MP2 & MP2 [sounds, file extension], vertex animations etc), so they would require some work in supporting such. And it is far too early for such.
It would be great at first to have replacement textures. Since we really can’t even expand the original textures with additional texture maps, such as light & normal maps. Because of the legal issues, probably drawing a light map for a copyrighted texture counts as a derivative work and gets us to jail. But instead of directly replacing all, textures would be the way to go.
But there are lot’s of other assets, such as the game data itself. So I think we require DK II for years to come.