Context
So, at my Session 0, one of my players (which will be playing a Barbarian) asked about gaining HP on leveling up. I intended to use the options on PHB - you either roll it or you get the average rounded up. I see concerns that I myself would like to change on this, that I'll comment later on. He stated that rolling a 1 on a 1d12 that only happens when he levels up (opposed to a damage 1d12 that happens every time he hits something) is frustrating.
He proposed that the players could roll it, and if it was below average, they could take the average rounded up. The problem is that the new distribution \$ x = \text{max}(x, \bar{x}) \$, where \$ \bar{x}\$ is the average from the hit dice would be increased by around \$ 20-22\% \$. That's significant. He said that it was used in another table he played, but the DM would rebalance the encounters.
By this point I should note that we are not in an optimization-heavy table, pretty much the opposite, but we are not playing Walk on the Park either and PC death is a thing.
Concerns
I agree with him - being a 2nd level barbarian that rolls a 1 on his Hit Dice is frustrating. Taking the concept of Goblin Dice, I don't think a roll that only happens 19 times in an entire full-leveled campaign should have high variance. It is not as game-changing as TPK on fail, but it is still significant, mainly early on.
The problem I mentioned about the PHB system is that rolling against taking the ceiled (rounded up) is not really a trade-off from a optimization point of view. It is less consistent and has a lower average [(d+1)/2 against (d+2)/2]. Specifically, rolling only makes sense for the suspense and gambling - but it is one of the rolls with least creative ways to use its outcome in my opinion. However I want some degree of randomness, not just "gain 7 HP".
Finally, the reason I instantly refused my player's method is because it would require me to rebalance every creature in the game in ways I don't even know how to (increasing their average damage by \$20\%\$ maybe? lol). Note: This question is not about this balancing. So, I don't want something that increases the average HP too much.
The question
I would like alternative ways to increase max HP when leveling up, consdering my concerns, summarized as follows:
- The mean average HP doesn't change by too much (I'd say about \$ 5\% \$) compared to the (1+d)/2 average.
- There is randomness. (i.e. it involves rolling 1 or more dice)
- The probability of low rolls (\$\leq d/4\$) is low.
If you have any experience with another method that works fine with my restrictions, I don't mind not having any statistics backing up, but please clarify the system used and any other house rules or DM concerns (like rebalancing encounters) that could interact with this change.
As always, frame challenging answers are welcome.