Skwire made Anuran so there's one already.
mouser also has PopUp Wisdom.
App has AnotherOneDone - a points based focus system and InstantBoss - a pomodoro like timer.
There's a Do or Delete program in one of the software directories.
Hell, you've done games yourself: http://timns.dcmembers.com/
I'm sorry if I'm being facetious but IMO it's alot harder to make a logic puzzle than to induce gamification. Up to this day, I still don't get sudoku despite reading some walkthroughs and seeing solvers.
You could take a cue from subliminal messages and WorkRave for example and combine blobbies with a countdown timer and a to-do list. The game concept being that it acts like a virtual pet. Your pet will only grow if you rest when that program pops up until the countdown timer shuts off. Similarly, the more you cancel the program - the more your pet dies. In terms of where the to do aspect lies - after the countdown is over, the task name is revealed and you can accept that/type a new task or switch to another task. if your pet dies - you must copy paste the task out of the program with the message warning you that you should put this in say a someday/maybe list and that you can't continue with the program unless you do so. (basically it just records if the action select text + ctrl x then ctrl v occurs)
For the volume meter, you could combine it with Skwire's Barbecue and have a progress meter that shows certain tasks only if it's set at a certain percentage. Like say... at 25% it will show only tasks (with progress bars) that you set to show at 25% and then when you slide to 100% - it will only show you tasks you've set to 100%. The idea being that you gain exp based on how much you've done a set amount of tasks that total to 100 in a set time chosen by the user. (i.e. 100% total in 24 hours = instant level-up for the first few levels and then it gets harder and harder after that)
For the logic puzzles, I really don't know logic, but you could do something like a reverse Goalscape. Instead of trying to increase the size of a circle, think of it like injecting tasks to build up a strong central frame. Say... organize each tentacle of a circle by estimated priorities and value kind of like Sudoku then the user must guess their own steps correctly by checking the actions in order. Worst case scenario, it doesn't motivate them to use it as a to-do list but it challenges the more logical among them to self-evaluate how good their outline is. Best case scenario, it trains them to act sequentially in a project.
I know it's a whole lot easier to state ideas than make them especially since I don't have the context of how difficult it is to make these apps nor am I exactly sharing best seller ideas that would guarantee it would increase someone's productivity but come on, you guys don't need interest. You are programmers. You've already unlocked the mindset of knowing you can spend time building something you want and there's a good chance you'll find you've built it or you lost interest. Interest are for us guys who can't make these on our own.
(Not even in our theoretical assumptions because we have no idea how codes look like.)
The bigger picture of why I'm saying this though is to emphasize how gamification is not a rigid category like developing a PIM or a timer or a Notetaker or a set of macro commands. It is purely a mindset preference, no different from making a windows app look like a mac app because you like seeing it that way. You don't have to be interested in gamification as much as you gamify what it is that you are interested in building in the first place. Sorry if this last paragraph is nothing profound. I just feel it needs emphasizing. (Note that this doesn't mean your gamified program isn't exempt from being judged as a poor game or poor program i.e. mouser makes a tower defense game where the enemies are tasks in a to-do list. It doesn't mean that if it plays like one of the best tower defense games it has succeeded in improving the motivation one has to completing their to-do list. Therefore it may get tons of praise and even be a hit as a paid program but it's still a failure as far as gamifying the program.)