I would like to see a new spell card called "carryover" or "remainder"; it would work like this: you cast it on an enemy, and if they are killed by the next spell, the "left over" damage points would transfer to another enemy instead of being wasted. for example, say your target has 10 HP left. you cast the carryover on them and then cast Ghoul for 160. the enemy dies and the remaining 150 damage carries over to the next enemy. or maybe it could be 50% of the leftover damage, like how some death spells heal for 50% of damage. this would be so useful in multiple enemy battles!
Interesting idea, but I think it would be a problem to implement. For instance, if there are three opponents and one dies with "carryover", how do you pick which one gets the "carryover"?
Besides, it would mean that every single spell in the game has to account for hitting multiple opponents. I suspect that would involve changes at the game engine level that would make the devs say 'Sorry, no can do.'
Your idea, though, has suggested an alternative to me: Spells with "splash". Currently, spells are either single-target or multi-target. The multi-target spells do the same damage to each target, except that modifiers like traps get taken into account individually for each target. These spells also typically cost four pips or more because of the massive amounts of damage they cause.
A "splash" spell would strike a single target for full damage and strike the enemies on either side of the target for half-damage. The pip cost would be one more than the "normal" cost of a spell of that rank. So, a spell that deals leprechaun-level damage to the main target would cost three pips instead of two if it was a "splash" spell.
The advantages of "splash" are that the values are fixed, and the rules are fixed. The targets are still somewhat variable, but they're at least easily determined.
In fact, I'll toss out a pie-in-the-sky idea - Instead of a class of "splash" spells, a new treasure card is created that modifies a single target spell in the same fashion that Keen Eyes and Tough modify a spell. Splash just takes a single-target spell and turns it into a treasure card that has +1 pip cost and causes "splash" damage.
That has a couple of benefits - It acts as a gold sink that also controls the number of Splash cards in use by controlling the purchase price. It also puts a limit on the use of the "splashed" card by making it a sideboard card.