Sylvaneth DON'T need Kurnothi, what they need is more Sylvaneth. Sylvaneth are easy to expand, they are based on plants. I can think of several new units from new treelords to new revenants. Such as, a treelord with a bow that shoots giant arrows(does artillery level damage) , a treelord priest unit of Alarielle , a treelord based on a fruit tree.
Revenant are also easy, from vine-revenants and thorn-revenants to even poison-revenants and nymph-revenants. We could even have something as simple as tree-revenants with bows and tree-revenants on beetle cavalry.