Pretty much nobody that I know of spends diamonds on buying plussing jewels for really low level equipment so let's get rid of them altogether.
Vendor Jewels should still be available for the gear around their zone levels. These should have their plussing rates updated to be a little more sane. 50% to +1, 33% to +2, 20% to +3, 10% to +4, 5% to +5 and 3% to +6. It'll make a great gold sink.
IS Jewels should simply be:
1. Basic Jewels ( Star, Moon, Sun ) = +1 to +6. Chances of plussing are Perfect to +1, 50% + ( max level - item level )% to +2. -10% to +3, -20% to +4, -30% to +5, -40% to +6. So if we're looking @ 99 gear w/ max item level @ 99... +1 perfect, +2 50%, +3 40%, +4 30%, +5 20%, +6 10%. Level 87 gear ( HOE ) would be: +1 perfect, +2 62%, +3 52%, +4 42%, +5 32%, +6 22%
2. Advanced Jewels = +7 to +12. Same chances as Basic. +7 is Perfect, +8 = 40% + ( max level - item level ), +9 = 30% + level difference, +10 = 20% + level difference, +11 & 12 = 10% + level difference
3. Enhanced Jewels = +13 to +16. +13 is perfect, +14 = 30% + level difference, +15 = 20% + level difference, +16 = 10% + level difference
4. +17 to +20: +17 is perfect, +18 = 25% + level difference, +19 = 20% + level difference, +20 = 10% + level difference
5. +21 to +30: +21 is perfect, +22 to +25 = 20% + level difference, +26 to +30 = 10% + level difference
Basically, the IS jewels would be more "powerful" on older gear and current gear is the gamble.
I would also suggest that RW/GF provide an IS item to "protect" the items from "breaking" ( losing a plus ). You could call them: weapon guard, accessory guard & armor guard.
Effect: 60 second buff that prevents the loss of plussing during failures. ( for each type )
I guarantee you that people would buy the crap out of them.