Ridley's Cohort said:
In real play I would expect any truly non-trivial negotiation for a 20th level PC will require the character to hit DC 40+ using those house rules. So a 20th level Sorceror who has his 30 Cha and 5 ranks of Diplomacy cannot conceivably succeed if the DM asks the player to pick up the die and roll. A Fighter who maxs out Diplomacy as a cross-class skill cannot usefully apply the skill except with the most minor NPCs.
That creates a peverse incentive to not take Diplomacy at all for most characters.
I can see what you mean about how the house rules could provide a disincentive to pick up a few ranks. That sorc will probably have a +8 Cha mod or so (16 base + 4 from levels + 6 from item), +4 with greater heroism (or other morale booster), +3 from a circlet of persuasion (not too expensive), for a total of +20. If you give a +5 Wis mod to a generic level 20 character (probably clerics will be higher, most will be lower), then the DC starts at 40, putting it outside the character's reach unless it's a very favorable deal for the NPC. If the NPC is at all hostile, the PC can forget about it.
I hadn't really considered the 'dabbler' case. It's tough to give a dabbler any real utility without making things trivially easy for the expert. Perhaps one thing that could be done is double the size of the favorability modifiers. -10 just really isn't a big enough penalty for convincing someone to trade their castle for a string (unless, of course, you tell them it's a magic string, in which case they're entitled to a Sense Motive check against your Bluff); after all, this means that a duke who's 75% likely to trade you his castle for yours is 25% likely to trade it for the string. Likewise, +10 isn't a big enough bonus for convincing someone to give you the time of day in exchange for a bag of money. This might give the dabbler a reason to grab some diplomacy without making it too easy for the master. Actually when I think about it, maybe it makes more sense to double the relationship modifiers as well and give the skill a base of 10 instead of 15.
Here's a few test cases for high levels, assuming you're always acting on a level 20 NPC with +5 Wis mod:
10 + 20 + 5 + 20 + 20 = 75: Convince your arch-nemesis to agree to a horrible plan. Who can do it? A 20th-level bard with 23 ranks of diplomacy, +12 Cha mod (18 + 5 from levels + 5 from wishes + 6 from cloak), +6 from synergies, +3 from skill focus, +2 from negotiator, +2 for being a half-elf, +2 for bardsong, +3 from circlet of persuasion, +4 from greater heroism, +1 from a luckstone, on a lucky roll.
10 + 20 + 5 + 10 - 0 = 45: Convince an enemy to agree to agree to a fair plan. Who can do it? A character with 23 ranks of diplomacy, +6 Cha mod, +4 from synergies, +3 from circle of persuasion, most of the time, or the diplomacy ultra min/maxer, all the time.
10 + 20 + 5 + 10 - 20 = 25: Convince an enemy to accept a very favorable plan.
10 + 20 + 5 + 0 - 10 = 25: Convince a stranger to accept a favorable plan.
10 + 20 + 5 - 10 + 0 = 25: Convince an ally to accept a reasonable plan. Who can do it? A character with 8 ranks of diplomacy, +3 cha mod, +2 from synergies, +3 from circlet of persuasion, most of the time, or the diplomacy expert, all the time.
10 + 20 + 5 + 0 - 20 = 15: Convince a stranger to accept a very favorable plan. Who can do it? A character with 1 rank of diplomacy, +2 cha mod, +2 from synergies, most of the time, or the diplomacy dabbler all the time.
10 + 20 + 5 - 10 - 20 = 5: Convince an ally to accept a very favorable plan. Who can do it? Krunk the loudmouthed, horrifically scarred half-orc barbarian most of the time, Average Joe all the time.
I think this might work out a little bit better. Any thoughts?