+1 per 4 levels would be fine with me. I want my character to improve at least a little bit over the course of 20 levels.
Well, one of their goals appears to be that monsters are useful at a much higher range of levels. One of the examples was that a couple of Orcs might be a challenge for 1st level characters, but a 10th level character might still be afraid of 40 Orcs.
The only way to accomplish this is to make it so that attacks and defenses over the course of 10 levels don't significantly increase.
Let's assume defenses NEVER increase and that Orcs have the same chance of hitting you at level 10 as they do at level 1(This also makes armor extremely valuable). You'll have approximately 5 times more hitpoints than you did at 1st level. So, in order to take on 10 times as many Orcs with about the same difficulty, you do need to defeat them faster. We do that with a slight increase in attack accuracy and a large increase in damage.
They have a 12 AC. The precon Fighter gets +6 to hit at 1st level. So, ideally, if you are still going to fight them at 10th level, your bonus to hit can't be so high that you hit them every time, or the battle becomes pointless.
So, let's assume you've acquired a +2 weapon by then. That brings your attack up to +8. I would think you couldn't have more than a +2 from leveling at 10th level. That way fighters hit on a 2. Meanwhile, the rest of the group might only have gotten +1 in 10 levels requiring a 4 to hit.
If you get more bonuses than 1 or 2 over the course of 10 levels, then low level monsters quickly scale away so quickly, they can't be used anymore.
The more I look at it, the more I think that +1 to hit every 5 levels for Fighters(Also Fighter-like classes such as Paladin and Wizards with spells) and +1 to hit every 10 levels for everyone else looks around right for scaling.
This means that at level 20, the difference in attack bonuses goes something like this:
Fighter +13
Wizard with spells +13
Paladin +12
Cleric with spells +12
Rogue with melee +10
Cleric with melee +9
Wizard with melee +6
The highest AC PCs appear to be able to have is 21. I would guess the same is true of monsters, and that most of the level 20 monsters will have ACs in the 19-23 range.
This puts them firmly in the suicidal range for most 1st level characters. Even a 1st level Fighter with +6 to hit isn't going to be able to defeat something with 350 hitpoints and an AC of 22.
However, it does allow more "realistic" scenarios where 100 1st level Fighters attack the monster and manage to do enough damage to kill it.
One 20th level Fighter being approximately the equivalent of 100 1st level Fighters seems about the right increase in power to me.
Then again, it's also possible they remove ALL attack bonus scaling...but since magic weapons still add to hit and damage, they become way more powerful than levels are. A level 1 fighter with a +5 sword might be the equivalent of a level 10 fighter in terms of damage output. Although, that would work to explain why magic weapons aren't bought or sold. If anyone who acquires a magic weapon suddenly has the power of a legion of warriors, they'd be priceless.
Although, it appears they are going for roughly parity in terms of how much your skill(+2 for proficiency, +1 for being a Fighter) and stat(They appear to be assuming +3 for everyone's prime stats) affect combat.
I would guess that in order to maintain that parity, they'd have to add a third pillar of "experience" and make it so you didn't gain more than about +3 due to levels.
