FWIW, personally, I would pay for no ads and maybe brief definitions of the words that I have just crossed out, but I would be turned off by microtransactions like boosts, extra life's or skins.
Word definitions would be awesome. Are there any good free sources for this? That also have offensive words flagged?
I'm not sure how to do a free version of the game that isn't so good that nobody upgrades though to be honest. With the competition on mobile now as well, I feel freeium is a must to get tractor too.