Second, Narset's second ability is really good. Consider this card:
Seal of Fork 2UW
Seal of Fork enters the battlefield with 3 Charge counters.
Remove a charge counter: Copy the next spell you cast this turn. Use this ability only once each turn.
I think that card is stupidly powerful.
I get what you, and some others in the thread are saying, but I cannot fundamentally get behind this kind of comparison.
Rebound != Fork
Forgetting about the "if it resolves" portion of the argument. Having to wait a turn to get your "copy" is not an insignificant thing. Yes if you get cantrips going with the ability, and start chaining things, it is a good engine for a few turns. And dreams of rebounding Time Walk and then next turn rebounding Regrowth is effectively a win if the opponent doesn't have an answer. I absolutely like those aspects of the card.
But vintage is a format that cheats 10 mana Enchantments into play, and I don't think anyone ever tried to SnT a Cast Through Time into play, which gives Rebound to all instants and sorceries you play for the rest of the game. I'm pretty sure the only crowd chomping at the bit to get their hands on that effect was the casual/EDH crowd.
Anyway, I'm not sure what to make of the card overall. I largely like the abilities, and I like the card. Is it vintage playable? I don't even begin to know (although it does seem to have decent synergy with Mentor). But Rebound and Fork/Twincast effects are not at all the same thing, and I do not think it's right to compare those effects as above. Unless the first thing you resolve with Rebound is Time Walk (which is probably pretty close to an insta-win for


)... A lot can happen between you saying "Go" and your opponent doing the same thing.
I feel like I need to add a disclaimer to my sig for a while... I know I'm still very new to modern vintage, so if I make a stupid statement in ignorance, I apologize, but outside of pretty specific Restricted cards, I'm not sure that giving a spell rebound is really that powerful when compared to Twincast or Fork (Or Wild Ricochet, for that matter). And some of the restricted cards will never want or need Rebound. Why would you give Yawg Will rebound? If you resolve the first, you should probably pretty much win, right? I've won 1 game where my opponent resolved Yawg Will, and that was because he pretty much straight up bricked on a Preordain and Ancestral when I was already ahead on board.
EDIT - Meant to take out the bit about Chandra, The Firebrand... It's not that relevant, really, so if you saw it, please forget I said it.