New Legacy Moves
Will there be new legacy moves? i.e. moves that are currently available that will become unobtainable once gen 3 is released? And if so, would it be a smart idea to use TM's now ;)
Answers
Nobody knows for sure, but it's likely to happen given that's exactly what happened when Gen 2 hit properly.
What moves will become legacy is anyone's guess really but, if you want to protect yourself and have the TMs to spare, I'd try and save at least one strong moveset of every top tier Pokemon, for example Dragonite's Outrage and Hurricane. Gyarados' Hydro Pump has been frequently speculated as a likely casualty as well.
Topic has been brought up many times on this site so I'm sure you can look back and find some fairly comprehensive lists.
Only thing you can do is have some of your Pokemon have the most optimal move sets: Example Gary with Hydro Pump or Tyranitar with Bite. Also look at Tier A for movesets that you wouldn't think of like Aerodactyl, make sure you have 1 with Ancient Power incase he loses that and gets a rock fast move. Thats all you can really do.
Just speculation. It is likely to get the new fast move Waterfall, but then it might also get its charge moves shuffled too. If it does lose Hydro Pump, it would likely get Aqua Tail, which is not too bad a change but just not the best moveset that it could have. If you have an extra Gyarados and TMs to spare, or a high IV Magikarp and the candy to evolve, it might be a good idea to go for Hydro Pump now.
It's also possible that DP will either lose damage (90 to 80) or take more time, and that CC will gain damage or get quicker. If it's CC at 120 vs DP at 80, now the two are pretty close, and CC might be the better move.
And of course, all of this is moot if either Machamp, Blaziken, or Hariyama get Focus Blast. That would the best fighting combination, period.