Source post compiled by fishers64, suggestions by many BZPers. Quotes of original suggestion posts included there.
Ba-Matoran (missing elemental trait)
Suggestions: Sensing of which way gravity is, jumping higher because of lesser gravity, “toleration of large pressure forces such as huge weight being placed on top”, being heavier, natural sense of balance, being heavier or lighter, resistance to gravity
The fanon Expanded Multiverse will likely adopt whatever is chosen in these polls as EM-canon, subject to final call by Swert and myself. I would also suggest that fans consider treating the winner as "headcanon" to use in fanfics, though ultimately such decisions are up to each fanfic writer.