Just something i started working on in advance the other month. In advance, because we still have an unannounced game to finish before this can get serious.
On the other hand, not being too deep into the production yet allows for another type of creative flow. These make ample use of unique characters/tiles and the cartridge hardware will just have to comply (ie have a rather big prg-rom). For example, the parallax effect in the second screen requires x8 as many tiles as normal for the treeline background. Thankfully, the cost of a 512kB prg-rom NES cartridge is close to that of one with less. Program size is not even the most significant cost factor, up to that point.