I was going to point this out too. Also note the placement of grass-Finn next to PB. I'm not sure if its foreshadowing a relationship where human-Finn gets replaced as he feared or simply symbolizing how grass-Finn hasn't gotten over PB like human Finn has.
Also note how going from right to left it's waifu's that Finn has pursued, ending at Huntress Wizard of course. This could allude to HW endgame.
I seriously do believe they are using grass or plant life in general as an allegory for change and growth, that growth is Finn getting over his hormones and no longer pursuing unstable "fiery" relationships if you will (FP), and relationships that are simply impractical (PB and FP); essentially "growing" into a natural relationship with HW, as well as growing out of thinking he's the protagonist in life (there's now two Finn's)
TL;DR huntress wizard cumfirmed, GRASSED confirmed>>91787866
This would be great but as I said they're probably going in a different direction with that.