Once I commit this, the only way to use it for those who want it ASAP, is to manually modify some XML files.
Add the following to game.xml (modifying the data if desired).
<autotileFormats>
<format across="3" down="4" id="0" name="RMXP Autotiles">
<autotile corners="2,4 3,4 2,5 3,5" flag="0"/>
<autotile corners="4,0 3,4 2,5 3,5" flag="1"/>
<autotile corners="2,4 5,0 2,5 3,5" flag="2"/>
<autotile corners="4,0 5,0 2,5 3,5" flag="3"/>
<autotile corners="2,4 3,4 2,5 5,1" flag="4"/>
<autotile corners="4,0 3,4 2,5 5,1" flag="5"/>
<autotile corners="2,4 5,0 2,5 5,1" flag="6"/>
<autotile corners="4,0 5,0 2,5 5,1" flag="7"/>
<autotile corners="2,4 3,4 4,1 3,5" flag="8"/>
<autotile corners="4,0 3,4 4,1 3,5" flag="9"/>
<autotile corners="2,4 5,0 4,1 3,5" flag="10"/>
<autotile corners="4,0 5,0 4,1 3,5" flag="11"/>
<autotile corners="2,4 3,4 4,1 5,1" flag="12"/>
<autotile corners="4,0 3,4 4,1 5,1" flag="13"/>
<autotile corners="2,4 5,0 4,1 5,1" flag="14"/>
<autotile corners="4,0 5,0 4,1 5,1" flag="15"/>
<autotile corners="0,4 1,4 0,5 1,5" flag="16-17,24-25"/>
<autotile corners="0,4 5,0 0,5 1,5" flag="18-19,26-27"/>
<autotile corners="0,4 1,4 0,5 5,1" flag="20-21,28-29"/>
<autotile corners="0,4 5,0 0,5 5,1" flag="22-23,30-31"/>
<autotile corners="2,2 3,2 2,3 3,3" flag="32-35"/>
<autotile corners="2,2 3,2 2,3 5,1" flag="36-39"/>
<autotile corners="2,2 3,2 4,1 3,3" flag="40-43"/>
<autotile corners="2,2 3,2 4,1 5,1" flag="44-47"/>
<autotile corners="0,2 1,2 0,3 1,3" flag="48-51,56-59"/>
<autotile corners="0,2 1,2 0,3 5,1" flag="52-55,60-63"/>
<autotile corners="4,4 5,4 4,5 5,5" flag="64,66,68,70"/>
<autotile corners="4,0 5,4 4,5 5,5" flag="65,67,69,71"/>
<autotile corners="4,4 5,4 4,1 5,5" flag="72,74,76,78"/>
<autotile corners="4,0 5,4 4,1 5,5" flag="73,75,77,79"/>
<autotile corners="0,4 5,4 0,5 5,5" flag="80-95"/>
<autotile corners="4,2 5,2 4,3 5,3" flag="96-103"/>
<autotile corners="4,2 5,2 4,1 5,3" flag="104-111"/>
<autotile corners="0,2 5,2 0,3 5,3" flag="112-127"/>
<autotile corners="2,6 3,6 2,7 3,7" flag="128,132,136,140"/>
<autotile corners="4,0 3,6 2,7 3,7" flag="129,133,137,141"/>
<autotile corners="2,6 5,0 2,7 3,7" flag="130,134,138,142"/>
<autotile corners="4,0 5,0 2,7 3,7" flag="131,135,139,143"/>
<autotile corners="0,6 1,6 0,7 1,7" flag="144-145,148-149,152-153,156-157"/>
<autotile corners="0,6 5,0 0,7 1,7" flag="146-147,150-151,154-155,158-159"/>
<autotile corners="2,2 3,2 2,7 3,7" flag="160-175"/>
<autotile corners="0,2 1,2 0,7 1,7" flag="176-191"/>
<autotile corners="4,6 5,6 4,7 5,7" flag="192,194,196,198,200,202,204,206"/>
<autotile corners="4,0 5,6 4,7 5,7" flag="193,195,197,199,201,203,205,207"/>
<autotile corners="0,6 5,6 0,7 5,7" flag="208-223"/>
<autotile corners="4,2 5,2 4,7 5,7" flag="224-239"/>
<autotile corners="0,2 5,2 0,7 5,7" flag="240-255"/>
</format>
</autotileFormats>
In resources/data.xml, find any single tiles that you want to be autotiles and modify them with the following optional attributes.
autotile="0" (0 is the id of the autotile format in game.xml. If this attribute is missing, it's a standard tile)
autotileMerge="243" (243 is the id of additional tiles that this autotile can merge into)
Add an autotile image with the name [TILESET ID]-[TILE ID].png to the resources folder.
So the way the autotileFormat works is like this
<format across="3" down="4" id="0" name="RMXP Autotiles">
This means that an autotile with this format (id=0) takes its graphic from an image that's 3 tiles across and 4 tiles down (the standard RMXP Autotile image).
<autotile corners="0,2 5,2 0,7 5,7" flag="240-255"/>
This shows the four corners of the generated tile, and where they come from in the source image, counted in half-tiles.
Since we're counting in half-tiles, "0,2" is the top-left corner of the tile at 0 across, 1 down. "5,2" is the top-right corner of the tile at 3 across, 1 down. Corners are specified in the order top-left, top-right, bottom-left, bottom-right.
"Flag" specifies when the autotile should be used. It's a collection of bit flags representing the 8 neighboring tiles. You add a bit to your flag if the autotile does
not connect in that direction.
CORNER_TL = 0x01; //1
CORNER_TR = 0x02; //2
CORNER_BR = 0x04; //4
CORNER_BL = 0x08; //8
SIDE_L = 0x10; //16
SIDE_T = 0x20; //32
SIDE_R = 0x40; //64
SIDE_B = 0x80; //128
So flag 240 = 128 + 64 + 32 + 16 = SIDE_B + SIDE_R + SIDE_T + SIDE_L.
This means that if a set of corners have the flag 240, those 4 corners will be used for any autotile that has different tiles to its sides, and the same tiles at its diagonals.
I've attached a test of RPG Maker style autotiles, with a fully working RMXP format, and a half-implemented RMVX format.
Since the xml autotile formats are difficult to write manually, if you specify how you want your autotile to work with example graphics, I can use a program to create it for you.