Sounds very similar to what we already enjoy right here on Hive with Hive Engine - and have been for some time

Games like Hive account@hashkings use the mutable portions of the nft to update the water required on a seed after it has been planted in a plot (the status of being planted is also a property of the seed nft, and having a seed planted in it is a property of the plot nft)

Hive account@dcitygame uses the lockedNft property (what you might call "sub nft") to load other NFTs into an NFT for use in upgrading and creating NFTs within the game

All the while, the immutable portions of these NFT guarantee that property rights exist and the NFT continue to hold some kind of value

And, of course, Hive is not an energy hog

Definitely, there must be some value in these smart nft, or phantasma wouldn't be going through the trouble to copy these features 😀