The only indicator I think would be cool is a stall horn. For example, you could fly at altitude until you stall and set the warning for that airspeed (after you recover of course). Then, while you are landing, the speaker on your transmitter would give an audible warning when you are close to stall speed. You could then throttle up, dive to gain airspeed, go around, etc. That way you have the advantage of the stall warning without taking your eyes off the plane.
Hummm... I may need to patent that idea! hahaha

It's probably been done or at least thought about, but as Spitfire222 noted there are trade-offs. I bet most of the experienced pilots just have a good feel for their planes speeds so that is not needed. But, if the price was right and there was no huge weight penalty, I would consider one for the first time I maidened a plane, just to be safe. Especially one that I had spent hundreds of hours building!!!