Experience and Leveling

Everybody starts at level 1.

You begin your story as a heroic so-and-so made from the mystical stuff of destiny, woven of the cloth of fate with the potential for greatness or ruin in equal measure. You’re about to sally-forth into the weird world already leaps and bounds more impressive and hardy than a mere commoner, but never more than a single well-timed natural 20 away from your final rest.

Advancement: Milestone.

Given that we’re looking at multi-level parties, and the potential for multiple DMs… I think a good rule of thumb is we should award a character the next experience level after they’ve completed a number of adventures equal to their current level during tier one and two play, and increase that number by say, 50% for higher tier characters.
A level one character will become level 2 after their first session. A level 2 character becomes level 3 after two additional adventures. 3 becomes 4 after their sixth total adventure, etc, slowing progress at 11th and beyond. And I have high hopes that we may actually eventually get there.

Tier 1 and 2:

Level	Total Sessions Played	Sessions to Next Level
1	0	1
2	1	2
3	3	3
4	6	4
5	10	5
6	15	6
7	21	7
8	28	8
9	36	9

Tier 3 and 4:

Level	Total Sessions Played	Sessions to Next Level
10	45	15
11	60	16
12	76	18

So basically, if we were to find some sort of miracle and get together every week for a year, anybody who attended every session would, with 52 adventures under their belt, be well on their way to level 11.

HP per level: Max. Every time.

When you level up, don’t roll your Hit Die to determine how much to increase your maximum HP. Instead, improve by the maximum amount rollable (plus your constitution bonus, as normal). Given the potential character level differences, player skill differences, and the increased lethality of hitting 0 HP (see Exhaustion is Convoluted and Stupid), this makes things a bit more forgiving for when those damage rolls start landing and provides with more of a buffer in adventure design.