Would it be unbalancing to fold the social uses of bluff and intimidate into diplomacy, but leaving the combat applications alone?
Social interactions: diplomacy (lying, intimidating, persuading) and sense motive
Imposing mild debuffs in combat: Intimidate (should it be based off another ability like strength now?)
Making people flat-footed in combat: Bluff (maybe based off Dex?)
Would this make diplomacy too good? Charisma even worse? Things better for fighters?
Look at things right now:
Diplomacy - making people do what you want by talking to them.
Intimidate - making people do what you want by yelling at them.
Bluff - making people do what you want by lying to them.
I can see diplomacy and bluff, but not intimidate. Make the diplomacy and bluff into a CHA skill and you'll free up skills for your Face. Change intimidate to STR and that gives your combat types a way to interact (though keep the social rules for it).
Sense Motive has always been an odd one. On the one hand, it is a way to keep Mr. Face from needing just a single skill. On the other, it is only used in this case. Make it part of the new Social Skill means you get down to one skill for anything reasonably social. That is up to you. Use slight-of-hand and DEX for combat aspects of bluff. Or name it something like Trickery. Rogues will still probably take it.
So you have a different skill for 'speaking and listening (charisma)', 'yelling and flexing (strength)', and 'tricking (dexterity)'.
No real change for fighters, less skills for the party's Face, easier time for high-skill types to become back-up faces. I wouldn't do it if everyone is the party is likely going to go with it; it is fun to make a character with no diplomacy or bluff talk to NPCs.