It forces you to start and end an idea within a time-frame or two bars. If you find yourself going over then work on making sure you can hear/think/feel what two bars is. Also, there’s the challenge of playing something memorable enough to remember what you just played in order to play it again! With the added challenge of now having to play that idea in a new key.
Take the last thing you play at the end of the fourth bar and use it to start a new idea.
Take the last thing you play at the end of bar two and use that to play a new idea in the new key. (so there will be no repeating of the first idea).
Try to never repeat an idea. Maybe you could try to make all the ideas unrelated. (a weird way of working on making your ideas more related), or maybe try to catch yourself when you feel yourself travelling down a familiar path and change direction. (tripping yourself up, if you like)