The code that builds the map using tiles is not available to work with. It's not a part of the scripts that can be edited.
Not saying it CAN'T be done ... you could have a script that takes an image of the visible portion of the map, fades out, then uses pictures (or sprites) to map bits of the image to it and makes them fall into place.
However, have you REALLY thought it through? I don't think it would look very good - it would make it obvious to the player that everything is made up of square tiles. It would be next to impossible to determine which tiles should go together (so you could keep a tree assembled and have it fall into place as a single piece).
I suggest you take a screenshot of the visible area of the map, cut it into pieces, and then use some program (maybe even RM, using pictures) to simulate what you're thinking, and make sure it DOES look good before asking a scripter to spend time doing it.