Pikachu's current moveset seems OK, but I dislike that due to having two Electric-type moves, it wastes a move slot.
-Anyway, Thunderbolt is nearly iconic with Pikachu, so that's pretty much guaranteed to stay.
-Volt Tackle is currently Pikachu's signature move, and said exclusivity of the move is apparently the main reason for the existence of Wild Bolt. So unless Volt Tackle becomes non-exclusive, that seems like it's staying as well. I'd hate for the new upcoming move to be Wild Bolt; it's terribly redundant for Pikachu's line and completely defeats the point of Volt Tackle's exclusivity.
-Iron Tail helps Pikachu trump Rock-type Pokemon, which Pikachu has difficulty battling with if they are part Ground-type as well. Plus, they spent quite a few episode focusing on Pikachu learning it and even bothered to enhance its animation in Best Wishes, so deleting it kind of detracts from the importance of those episodes. But it's a relatively new move and many Pokemon can learn it, so it has the chance of going.
-Quick Attack is learned by many Pokemon as well, but I feel that to this day it serves as a reminder of the episode Electric Shock Showdown, where Pikachu's speed was an integral part of Ash's tactics, to the point where Brock said something to the effect of "Lt. Surge evolved his Pikachu right when he got it, so it was unable to learn the speed attacks that it could only learn at the Pikachu stage". Since the episode tends to be a memorable one (as Ash's Pikachu trumped his evo) and is one of the main driving factors for the Team Rocket trio hounding Ash to this day, removing Quick Attack would be a very poor decision IMO.
So, I don't think Ash's Pikachu should learn a new move for a while, though Surf and Fly could be welcome changes for one-time-only events.
As for the rumored upcoming new move in Best Wishes that Pikachu will use, I'm liking the Lightning Strike theory, despite its exclusivity to Zekrom. Zekrom also has the signature move Cross Thunder at its disposal, and the anime has shared exclusive moves among Pokemon before as well (see Raticate learning [Hi] Jump Kick in Battle Aboard the St. Anne, a move previously exclusive to Hitmonlee). Ash and Pikachu can also devise improvised moves such as Thunder Armor again with little effect on Pikachu's "moveset", so there's always that option.