So, co-incidently I started a Rise of the Runelords game a couple of weeks ago using what I dubbed a "3.75" ruleset. Since the announcement/release of the Pathfinder RPG, I've incorporated as much of it as I felt I could.
Firstly, as the players have already created characters, we *aren't* using any of the races & classes (though in general I think they look good - especially in A1.1). If/when characters die I might consider letting them make replacement characters from the Pathfinder RPG rules... but will have to see how they balance in a mixed 3.5/PRPG game first.
So what am I using? As well as a few 4e/other rules (action points, best of two ability mods add to saves, +1 to two ability scores every 4 levels), I'm using the following PRPG rules wholecloth or modified:
1) Starting hp bonus = Con score. If the characters hadn't already made up their characters I'd probably use the racial bonus rules though - they seem better.
2) Modified skills. I'm using Epic Meepo's suggestion from the "[Design Focus] Skills" thread, along with a heavily modified skill list (some from PRPG, some from 4e, some of my own invention):
Acrobatics – Dex (replaces Balance, Escape Artist & Tumble)
Appraise (used instead of Spellcraft to identify magic items)
Arcana – Int (replaces Knowledge[arcana] and Spellcraft)
Athletics – Str (replaces Jump, Climb & Swim)
Bluff
Craft (replaces Forgery)
Diplomacy (replaces Gather Information)
Dungeoneering – Wis (replaces Knowledge[dungeoneering] & Survival in dungeon areas)
Endurance – Con (replaces Concentration & endurance related rolls – class skill for classes proficient in medium or heavier armor)
Handle Animal (includes Ride)
Heal
History – Int (replaces Knowledge[history] and some uses of Knowledge[nobility & royalty] & Knowledge[local])
Insight – Wis (replaces Sense Motive and has additional uses)
Intimidate
Scholastics – Int (replaces Decipher Script & Speak Language – for every 2 ranks, round down, speak one new language – and provides ability to research)
Martial Lore (see Tome of Battle)
Mechanics – Int (replaces Knowledge[architecture & engineering], Disable Device & Open Locks)
Nature – Wis (replaces Knowledge[nature], knowledge[geography] & Survival in wilderness areas)
Perception – Wis (replaces Spot, Listen & Search)
Perform
Profession
Religion – Int (replaces Knowledge[religion] & Knowledge[the planes])
Stealth – Dex (replaces Hide & Move Silently)
Streetwise – Wis (replaces some uses of Knowledge[local] & Survival in urban areas)
Trickery – Dex (replaces Disguise and Sleight of Hand)
Use Magic Device
Note: Use Rope is no longer a skill.
3) I'm using the Combat Maneuvers rules wholecloth.
As the campaign progresses, I'll post here and/or in the specific area with updates on how the rules are working out. So far I've found the bonus starting hp make a real difference. The characters don't feel as fragile as 3.5 1st level characters and this in turn makes the players more likely to take risks. I think possibly Con score gives a few too many hp at first level and (as mentioned above) if I could do it over I'd use the racial bonus hp rules from PRPG instead.