Basing it off of Turn or Rebuke Undead effective level would probably a good way to go if it were not for the slew of items that boost it...there's all sorts of abuse possible there.

Making it "reliable" would boost it a little, but if you're only rolling 5d6 (because there's very little point in taking Cleric beyond 5th level), it's still not going to be anything to shout about.

Basing off of caster level or adding 1/2 all other levels (or similar) doesn't seem right to me, either...why should my Domain power improve if I'm focusing on martial or skill-based abilities? My Sneak Attack as a Rogue doesn't improve when I take Fighter levels, so why should Domain powers do the same?

About the only boost that really makes sense to me would be to allow Prestige Classes that either require the Death Domain (e.g. True Necromancer) or are clearly un/death AND Cleric oriented (e.g. Master of Shrouds) to stack with Cleric levels for purposes of the Domain power. This would have to be judged on a case-by-case basis by the GM and still wouldn't be a massive bonus.