This just gave me an idea for creating a map for a world (normally I just scribble some basic shapes and build something from that). Take 30 or so basic land cards shuffle them together and then deal them out into a grid like 4x4 or 3x5 and then convert the mix of islands, forests, mountains, etc into a map. Islands could represent any large body of water if falling in the centre of a land mass.
On a side note, Aurelia is CR 23 in 5e. Her stats are in the ravnica guide.
I pegged her at CR 18.
In magic terms casting cost 6 is generally at the upper limit of what is playable and things like Dragonlords tend to start at CC 6. Casting cost 10-15 is generally the upper limit of MtG critters and would be CR 20+ IMHO as they are basically epic level foes (epic being 15+ in 5E, 20+ 3/4E).