I couldn't agree more with the above post. Plan, plan, plan.

I've just begun reworking a large-ish system that I didn't write. I wish I were in your shoes instead of mine in this case because I first have to go in and dig through a ton of (poorly written) code to find out what it does. You at least have the advantage of knowing this already.

When you do get around to writing your new code, though, please take into consideration that next guy who will have to rework your code. Document everything and use coding techniques that are easy to follow.

And most importantly, Good Luck! Your ideas for the new app sound intriguing. Makes me want to go write some code myself. :)