I've noticed a few people rolling with a "safety net" of minimum half die size. Just for interest, here is the average of a die rolled in that way:
d4 (2.75)
d6 (4)
d8 (5.25)
d10 (6.5)
d12 (7.75)
Especially for the larger dice, that's a noticeable boost!
I prefer linear progression. Keep things simple.
Also: I HATE D3'S!!!!! If it doesn't come in your average little box of seven dice you can pick up for 5 bucks at a gaming store, it shouldn't be used in core D&D. Having to divide a dice roll, or get specialty dice for a core element of the game should not be necessary.
A while ago, my players complained about the flat randomness (no bell curve) of the HP roll
No. How would it be? Every individual level has the same odds of being a low number. That is, once you've gotten your first 3 levels and incidentally rolled low, once you gain that fourth level, the odds of rolling low are exactly the same as for the second and third level which you've already rolled. There's no actual dice karma, that's a superstition. So your fighter, who has 17 HPs at level 3, for the sake of the argument, has the same odds of rolling a low that the Paladin who rolled well so far has, even though the Pally has 31 HPs. In theory, the two gain the same average, and the Fighter remains theoretically behind the Pally if we assume average rolls from there out. The Fighter could get lucky, or the Pally unlucky, and they could even out, but there is no automatic fix for rolling low. It's all the luck of the roll.Hang on... isn't that pretty much a bell curve after 4-5 levels?