I don't know how much of a need there is for a pond care forum but I think it would be nice.
I have been doing my own research trying to find plants for my pond that will grow tall and help shade my pond/ fish in the hot summer. It would be nice to have a area to ask questions, share pictures, and talk about pond care.
From the threads I've read on ponds and other water gardens, it seems that many of their creators are vitally interested in the welfare of native species, both flora and fauna. Perhaps "Gardening with Native Species in Mind" would work? Or, maybe, the "Wildlife / Critters" forum?
And, of course, "Landscaping," as already suggested.
A read-through of the contents of these forums might reveal helpful information.
It dosent have to be fancy Maby add it to the Japanese garden area on the forum.
It can always be modified later depending on how it works out and how popular it is.
Seems that we have everything else on this forum, might as well have a pond/ water garden area too.
A tropical plants forum and/or orchid forum would be nice as well. A lot of hard-core orchid enthusiasts would probably benefit from an orchid forum (and more people might might join).