What is the reason for wanting the extra point less of a sure thing? To get more variability in the score and thus more uncertainty and drama?
A few things. First of all, the automatic XP is just bad. We waste our time watching a play where something interesting happens, what, once a year, maybe? Given that, there's a few things you can do.
1. Get rid of the XP entirely. I'd be fine with this.
2. Move the XP such that the odds are more like they were 20+ years ago (90%). This makes the play itself at least somewhat interesting, and has the added benefit of promoting going for 2. My philosophy is that all real football plays are better than all kicking plays, so this is simply a good thing.
3. My favorite: get rid of the XP, give 7 points for a TD, and allow the team the option of gambling a point on a try from the 2. They make it, they're up to 8. They miss it, they're down to 6.
I often saw people complaining about how they shouldn't "change the game." But the fact was that the game had changed, even without a rule change. Kickers are much much better now than they used to be.