Make it easier to exchange data with other services (API)
Ideally, one could sync (both ways) with their favorite "other" service (my favorites in this case are Hiveminder and Trello), but it seems to me it would be a lot of work to cater to absolutely all user wishes.
What is needed, for example, in case of zapier.com is for someone to make the "zapier app" of WeekPlan: zapier.com/developer/ - it seems easy as long as you already have a RESTful API and a couple of other preconditions.
However, if there are currently no plans for a simple API to WeekPlan, then what I'm effectively asking for is exactly that - an API. Preferrably to the specifications of zapier.com/developer/perfect-api/ or similar.
The beauty is, when there's a simple API to your service, others can help with the other steps. For example, I'm already thinking trying to do one Zapier app for two services that *have* APIs (disclaimer: I'm not a professional developer, so this will be an exercise and a learning curve for me, I'm sure).