Many people have most likely thought over the idea of turn signals (indicators) for bikes, and what design could be effective and practical. LED's and lithium-ion batteries make such an objective more obtainable than it's been in past.
I've tried to think of a design that would enhance the effectiveness of conventional hand signals with an illuminated sequential display. A series of maybe 5 LED's lined along the trailing edge of a gauntlet...maybe 10" in length. The display would be activated when the wearer raised their arm to indicate a turn, one light at a time in sequence starting with the LED closest to the wearer's body and proceeding to the next in the direction of the turn until all LED's were lit, at which time the sequence would repeat. The effect would try to resemble a kind of illuminated directional arrow.
There would be the question of figuring out a location for the battery, and cables that would be the least annoying to the user. It would be a challenge to devise a gauntlet that wouldn't be hot and uncomfortable, and would hold the LED's where they're supposed to be so they could be seen by people on the road. Maybe the best that could be expected would be a cold climate, Fall-Winter, dark months of the year type of affair.