According to
@marni1971, green are the easy pads and yellow and orange require more work to use. The yellow pad south of Space Mountains seems to be off the table since they expanded the retention pond added retention walls.
Red and purple are armchair speculation. Red is a little bit easier and purple much harder, since it would require big changes in water management and maybe demolishing It's a Small World (we can only hope!). Access to orange, red, and purple is very difficult since the placement of HM, RoA, BTMR, and the train leave almost no access to that back area. One of them has to drastically be moved/altered.
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