I was looking through the Player's Companion and noticed that Hardy People costs over twice as much as Divine Blessing despite having less than twice the benefit. After seeing this, I was unsure how to add increased damage as a class power to a fighting 2 class. Would +2 damage cost 2 custom powers or should it be higher? Similarly, can other benefits be increased linearly and, if so, should there be a higher cost associated with having a greater bonus or a simple cap on benefit strength?
Others who frequent the forums can provide a better analysis of how much your proposed power should cost. However, an overarching principle of class building with the PC is that it’s not hard to build over-powered classes. Stacking abilities and high degrees of synergy often require some “Kentucky windage” to balance appropriately.
My gut instinct, though, is that a doubled fighter damage bonus will feel very OP before too long, unless you’ve made it harder for characters to have an 18 Strength and/or magical weapons.
In general it’s 2 for +2 damage. But... as highlighted above that is very strong. ACKs generally does not give out damage bonuses. Damage tends to cause things to die faster and trigger cleaves. I have built a class that gave +2 damage (ftr2), representing a large creature; but gave it a CON prime to avoid higher STR. I also gave it some indirect side abilities to water it down a little.
There is a lot of personal ‘feel’ in custom classes, ie does it feel right for your game? I hope this and the above post help.
For what it's worth, I have a class (well, race technically, but same idea at this point) that has a scaling damage bonus from custom powers in addition to any fighter damage bonus they might get. A player is playing it in my campaign, and simply put, it's overpowered. It's not gross enough for me to feel the need to change it mid-campaign, but I would change it before any sort of sharing or release or anything like that.
Without drawbacks added to the power (like fighting fury), I think my inclination would be to cap any custom-power damage bonuses at +1 if universal, +2 if situational, and I wouldn't want to see a +2 before 7th level or so. (Note that I would count 'all melee attacks' or 'all missile attacks' as universal.)
Multiple abilities centered around a theme is definitely the right call. Also, avoiding stacked damage completely may be a wise move based on your advice. The new version of the class is Fighter 2 HD 1 Thief 1 with graceful fighting (blade dancing), swashbuckling, skirmishing, and devastating (+1 to hit and damage). The two trade offs were armor limitations (unrestricted to narrow). I would write more about this class but that seems beyond the scope of this topic. Thanks for all the feedback!