Hi! Yes, unless something changes all the Elemental Dungeon tilesets will be released as DLCs. Part 3 (Dark, Light, Lightning, Metal) is scheduled to be released soon, I'm actually making the Steam materials right now. As far as more tilesets, we don't currently have anything planned in that EXACT series, however, there are plans for a similar and possibly complementary set coming up that might help. We're hoping to release the first set soon, possibly late next month, on our own channels. Degica may or may not release it on Steam, I have yet to propose it to them.
Hope this helps!