Your wordlist seems a little.... restrictive? I couldn't get any word ending in 'S' to be accepted - SINGS, FORTS, SEEDS, SPIES, ... as well as some others e.g. PRISE.
Ah, I had no idea that U.S. spells prise with a "z"! But surely SINGS/SEEDS/SPIES are present tense verbs as well as plural nouns? If you're going to keep an intentionally-restricted dictionary, perhaps it might be interesting to have it _only_ populated with singular nouns? That way it feels like each step is a transformation between objects rather than words?