I don't have a lot of free time, but tile art interests me, so I'll see what I can do. Let's continue this conversation on Discord; it's better optimized for back-and-forth messages. I often check the Polished Crystal server, so join and PM me.
In general if you can provide examples of what you're picturing—photographs, or better yet, Gen 3 or 4 tiles I can just tweak and devamp—that would be great.
Anyway, "silver/gray stones for a border along the water" sounds kind of like the cobblestones I drew for Ecruteak Shrine. Are those usable, or did you have in mind something the size of the default shoreline, 8px instead of 16px?
Trees are more difficult; for the cherry blossoms I just pasted flowers onto a recolored Ilex Forest tree. Other trees are devamped from FRLG and HGSS. Do any of them look like a good starting point? Maybe this, the sakura tree with whiter leaves?
Crystal Dreams is a great concept, and I hope to see you succeed at it. It might be a good idea to start your own Discord server and find early fans, beta testers, contributors, etc, if only as a central location for your material that's more flexible than a forum thread.
Good luck! ^_^