There is a lot of potential for Letternum to be a decent RPG or maybe an action rpg type game in my humble opinion. Just creating a rpg leveling progression mechanic that increase some sort of stats each level, some item(s) that effect health, keys to trigger door openings when in inventory and/or maybe a weapon that increases damage when equip, and maybe some choose your own path -pick this door for replayablity and you'd have a solid game that would peak a lot of people interests.
Again just answering the question and not implying there is anything wrong with the original whats so ever. If you made this based around a simple theme, and a time limit, imagine what you could do when untethered! The idea and simplicity of the letters and numbers is a unique approach and I believe you might be surprised at the interest a game like this could bring. You have a solid base to work on and the clicking mechanic will transfer over to mobile flawlessly. Hope this helps!