When I claimed that there was another Shard involved, it was based on my reading of the statement
"3 of 16 once ruled, but now the Broken One reigns."
My interpretation of this was that there were three Shards on Roshar, all 3 of which were generally benign (or at least natural for Roshar) and operated within their sphere of influence, doing Shard things. Then, just like on Sel, Odium (the Broken One) showed up and began wreaking havoc with their system, resulting in the desolations and the general war that is apparently ongoing.
Part of this is that I don't want Odium to be Ruin part deuce. Ruin was a natural part of Scadriel, even if he and Preservation had to be kept carefully balanced for the world to be amicable. Ruin's power, held by Sazed, is vital to keep Scadriel running. Odium is being built up as something else altogether, as somebody who is personally evil, not just driven to it by his very nature.
Another part of that is that it makes the story much deeper, and gives us many more delicious magic systems.
So I don't have a lot of information to back this idea up, but I haven't seen much against it either. If there are any interviews *ahem* which might shed light on it, I might be willing to read them, though.