While I'm not absolute on the notion that 5 letters is perfect, I found the current game lives on quite a precarious balance; There is a Japanese version with around 65 possible letters, and the secret word being 4 letters, but this turns out to be significantly harder, with the game rules adjusted accordingly to allow up to 12 tries. Despite being able to play it in my mother tongue, I found it less fun. Tweaking the formula (even in English) will likely require lot of thought into re-balancing.