If Fiddlesticks E procced onhit effects then suddenly Runeglaive is a good item on him.

It seems like a rather simple solution. Obviously they might need to adjust the ratios a bit so that a laning fiddle can't lichbane nuke someone with just E; but really that's the only thing stopping Runeglaive from working on him.

Honestly though, people are really exaggerating how bad runeglaive is on fiddle, it's not like fiddle never auto attacks. Generally you ult in, use your silence, then wait for your silence to end before fearing them, then using your drain as a finisher. There is plenty of time for an auto attack in that combo.