Hmm since I really didn't create many tiles yet I can't give you any advice about it. But I do like editing them, recoloring, rearranging and stuff. You can create new stuff by just putting two rtp tiles together and edit it a bit, draw a bit on it, connect it, that's fun~
I either try to make games in completely rtp (but also tend to use stuff I find on the internet/here) or I make the whole game stylized with simple pixelart (idk what it's even called, fe these old pokémon graphics for gb) but since I'm lazy and perfectionist I mostly just stick to the rtp. It might not even seem like it but there's so much you can do just with the rtp and especially with the mv one. People won't ever believe XD