I know this topic has been done to death and I know you've all heard it, but I'll write it anyway. I really want Gyarados to return, and If it does - change it to a FREAKING water/dragon type like it is supposed to be!
Personally, I have always HATED the water/flying type. Adding flying properties to water types sure did give them an immunity to ground and neutrality to grass, except now it has a 4x weakness to electricity and is now weak against the most annoying attack in the metagame - stealth rock. You know what? I wouldn't find Gyarados' typing that bad if IT ACTUALLY LEARNT ANY FREAKING FLYING MOVES! Giving Gyarados a STAB boost for moves that it doesn't even have is one thing I hated.
The thing I hated most about Gyarados' typing is that in gen 1 it was originally going to be a water/dragon. But since this typing makes it only weak to dragon, and the only offensive dragon move was dragon rage and twister (I think), GameFreak decided to change it's typing so it wasn't overpowered.
But in the later gens they added more powerful dragon moves (such as dragon pulse an outrage), but GameFreak still left Gyarados as a water/flying. WHY!!!!!!!!
With all honesty, I would even prefer Gyarados to be a mono - water type, if of course it doesn't become a water/dragon in gen 6.