Would it be possible to popup a reminder to close a flightplan? Could be triggered when plane is on ground and standing still or when closing Navigation. Normaly I fly without flightplan but when crossing borders I file a flightplan. If I land on an uncontrolled airfield I could forget to close the flightplan(just did). They send the police to check if the plane had landed an next day I got a lot of angry people in my face.

I wasn't lying before. SkyDemon really doesn't know you're flying on a flightplan. The fact you have previously filed a VFR flightplan for a route (on that device, or any other) is not known by any particular device logged in to the same SkyDemon account.

Our planning warnings and suggestions are very different to the new class of alert you are proposing. I just don't see how and where it would fit in. You may know some other people who think it's a good idea, but unfortunately nobody else has ever suggested this. If they do, we can start thinking about the best means to make such a thing accessible.

One possibility would be to use the device's internal alarm system either by entering an alarm via the clock app, or maybe it's possible to set an alarm from within Skydemon.

In logs there is a Log Stop time - the reminder could be popping up at that point - if the option for a reminder is activated.

Or just define a timer (Navigation options | timers) in SD. Not as comfortable as your approach, but it's already implemented.

Edit: After rethinking my proposal I believe it's not as good an idea as I thought in first place.