Well I have the feeling that they kinda planned on Finn having a final love interest who has the same interests than him, who also loves adventures. Just remember how he wanted to clear a dungeon with FP when they were a thing.
I also think Marcy was kinda that way but there was this "she is too old for him" thing I guess, just like what they did with Bubblegum.
In the end after all this suffering and MarcyXBonny shilling, they didn't really wanted to write a proper love story for him and just decided which fem in the series would be adventurous enough to be with Finn and that's it.
I'm not against HW but there wasn't enough character development or story behind them.