I think introducing new kinds of ways to balance spells is fine. After all, if you start looking at all the supplements out there, spells start to look a lot alike since they all work on the same rules. New rules=new kinds of spells=possibility of a good thing.
And I wouldn't even say the extra partial action is the reason haste is unbalanced. What I mean is that its not getting one more action a round that's the big prob. Its the fact that there's a huge difference between a standard and a fullaction. The fact that I can take a moveequivalent or standard action, and still get the benefits of a full round action is huge especially at high levels. The move in, and then take 4 attacks for a fighter is one example.
With that in mind, I would go with the haste kicks in one the next round solution. That's how I used to play haste until I understood how it actually worked, and I think its a great compromise. It adds the oppurtunity cost mentioned above, and since high level combats are usually very quick, then the player has to seriously consider it. It becomes a useful spell, not a no-brainer spell.
If that doesn't fix it (but I think it will) then the delay idea should be tried.