You don't need to include , since reducing a card by a colored mana can also apply to the generic cost. Right now this is a 5c color identity, but I think you could write it as "Choose a mana type. The next spell you cast this turn costs one mana less of the chosen type. (The mana types are ...)" Then the card has a colorless identity and can go in any commander deck

So this is, roughly, to be cast in response to a wrath effect - either yours or your opponents, and it lets you get off a single big spell?

Or you can use it much earlier in the game to get a mana or two back for a smaller spell. Which is less flashy, but probably actually more efficient.

That it lets you get off-colour mana is weird. Most cost reducers only give you one colour - so that cards that cost double coloured-mana remain hard to splash. This gives an interesting route to cast kind of ridiculous things.

Definitely worth a try.

As for keywording it? Interesting choice of word - but more to the point, I'm hard pressed to find many effects small enough that it's safe to copy once for every creature that dies. "Gain 1 life" "Deal one damage" "You may pay to steal one life" The most amusing one I can find is "It's controller may return it from the graveyard to play" for pseudo regenration.

I guess there's also the possibility for really big effects once you get to 7-8 mana for the initial casting.