First, a little about D&D Next's Expertise Dice and Pathfinder's Stunning Fist feat.
A monk doesn't need to meet the prerequisites of Stunning Fist, gains it for free and can it use once per day for each monk level (other characters must meet all prerequisites, buy the feat and can only use Stunning Fist once per day).
Well, what does that have to do with the Weapon Focus and Weapon Specialization feats? I believe these are the Fighter's "hallmark" feats (and usually every PC Fighter that I know buys both). If so, why are they so boring? (I admit that I hate "arithmetic feats"... like 99% of D&D 4th feats).
The idea here is to give Fighter some exclusive toys when buying those feats. It’s here that the Expertise Dice come in. For those of you that don't know them, Expertise Dice (or ED) are a class feature of D&D Next. To be sincere, I don't know if they still are called that. I don't follow D&D Next's open playtest anymore.
ED are an amazing idea with a rather poor (or clunky) execution. Basically, they're a floating dice poll to be used every round and added as a bonus to things like attack, damage and AC. They were supposed to be the Fighter's "unique stuff" for Next, but by last time I checked
almost every class used them. To make things worse, there's a Maneuver System attached to them. In the end, their implementation ruined D&D Next's "simplicity" that I so enjoyed from the first packet.
I still believe that ED are great (and I'll probably use them for my OD&D games). In a sense, they remind of the Action Dice of Dungeon Crawl Classics (in fact a lot of stuff in D&D Next reminds of DCC RPG).
They can be a good "solution" for the Fighter's hallmark feats in D&D 3.5 and Pathfinder. This is how I'd use them. As usual, this is pure brainstorming without any playtesting (sorry, no time).
Weapon Focus feat
A Fighter who buys Weapon Focus gains a Focus Die (FD) instead of the normal benefit of the feat. During a combat, at the beginning of her turn, she can roll her FD and add its result to one of the options below (only while wielding her selected weapon):
I. As a bonus to attack rolls.
II. As a bonus to combat maneuver checks.
II. As a bonus to AC.
IV. As a bonus to CMD.
The Fighter must declare where in what option she’s using her FC before rolling. All bonuses last until the beginning of the Fighter’s next turn (optional rule: rolling dice is fun! So, the GM is allowed to let the player roll her FC at every attack roll – or against every attack, if using for AC –, instead of rolling once and using the result to all rolls for the round).
The FD beings as a d4 at 1st level, increasing by one step every 5 fighter levels (5th, 10th…), to a maximum of 1d12 at 20th level.
Greater Weapon Focus feat
You gain two Martial Dice (MD). They work like FD, except that once used they’re spent until the beginning of the Fighter’s next turn. You choose to use a MD after rolling but before the Gamemaster narrate the consequences of your action.
MD use the same type of dice determined by your FD. If your FD is a d8, you gain 2d8 as MD. It’s recommended that the FD be of a different color, so as not to be confused with the MD.
MD can be spent/distributed on the following options:
I. One or more Martial Dice can be spent and the result added to one attack roll or combat maneuver check.
II. One Martial Dice can be spent and the result added as a bonus to AC against one attack.
II. One Martial Dice can be spent and the result added as a bonus to CMD against one attack.
IV. One Martial Dice can be spent to automatically confirm a critical hit.
V. One Martial Dice can be spent to negate one Attack of Opportunity.
VI. One Martial Dice can be spent to increase by 5 ft one attack roll or combat maneuver check (your threat range doesn’t change).
VII. Two Martial Dice can be spent to redirect an attack from an adjacent target to you.
A Fighter of 12th level gains 3 MD from this feat, plus another die at 16th and 20th levels (a maximum of 5 MD).
For example: a 16th level Fighter with Greater Weapon Focus has a 1d10 as FD and 4d10 as MD. While using her selected weapon, before rolling, she decides to use her FD as a bonus to attack rolls. During her turn, she spent one MD to negate an AoO due to moving. She then spends another MD as a bonus to one attack roll, holding her final MD to boost her AC if attacked.
Weapon Specialization feat
A Fighter who buys Weapon Specialization can choose to add her Focus Die (FD) to all the damage rolls. When a FD is used on damage rolls, it’s open-ended (if you roll the maxim number on the dice, roll again and add it).
Design note: yes, you must choose to use your FD on damage instead of using one of the 4 options granted by Weapon Focus. Non-fighters get the +2 bonus to all damage rolls. I don’t find to unbalance the feat, because when a fighter decides to use Weapon Specialization she tends to deal more damage. Besides, fighters in Pathfinder (due to Weapon Training) always deal a lot more damage. Finally, the open-ended bit above provides a good boost (and I hope an interesting gambit element).
Greater Weapon Specialization feat
You can spend Martial Dice (MD) on damage rolls. These rolls are open-ended, except that only die can “explode” (if you roll, for example 2d10 and come with 2 “10”s, choose one and roll again).
You can also spend one MD to treat you next attack roll as magical, silver and cold iron. You can spend 3 MD to make your next attack bypass alignment-based DR. Finally, you can spend 4 MD to bypass adamantine-based DR.
P.S.: I wrote these rules about a month ago. Reading it again now, just before posting, made me note a possible design mistake – these feats should be class features (because the Fighter is still forced to buy them). This is easy “to fix” if you’re playing D&D 3rd or 3.5. In Pathfinder thing are trickier. That’s because I should have find a way of using Expertise/Martial Dice on the Armor and Weapon Training class features. Why didn’t I do that? Well, first I’m lazy; and second I don’t have much free time lately. To tinker with those class features would require more thought. It wouldn’t also change the way those feats work for Fighter – they’d still suck. Anyway, if I get a better idea on how to use Expertise/Martial Dice on the Fighter I’ll let you know (until that time, you may be interested in my variant Fighter that also has a resource-based mechanic).