Meh, as part of the attack action it's confirming works well enough. That should cover AoOs and any other oddities while still tying it to a single attack roll. If you still feel it's been too heavily nerfed from your original vision, you could always just make it a (spell compendium) hunter's mercy clone, discharging to make an attack threaten critical if it hits. This brings it back to being ideal with a scythe, though.
I wanna spoil stuff!
As for whether or not a free crit is too much -- it's really not, but D&D screwed that up for us. Between the vorpal enhancement and auto successes, 20s simply carry too much weight to be handed out. Saying "automatically deals normal critical damage if attack succeeds" really even isn't so bad. But that 20? That's the sort of stuff you see in Deities and Demigods, with "automatically win" scenarios, and then specific-vs-specific verbiage as to whether your immovable object can compete with my unstoppable force. From here, the next logical step is the Immortals stuff with +8 infinities attack or whatever.
On the topic of free crits, could be fun to make a disciplined crit fishing fighter loaded with special crit effects over damage -- upon critical, opponent is slowed, cursed, and you gain half the damage dealt as temp HP. Not a bad deal.