Er, dude? Your "start" is already in the game: TP over a 100% threshold proportionately improves the WS by a percentage of the difference made by the next 100%. As a very basic example, take the Weapon Skill Seraph Strike. "Deals Light elemental damage to the enemy. Damage varies with TP." At precisely 100% TP, the Weapon Skill gives a x1.00 multiplier to your damage; at the 200% threshold it's x2.00, and at 300% it's x3.00. If you used Seraph Strike with 117% TP, however, the damage multiplier will scale up 17% of the additional damage provided at the 200% threshold, resulting in a damage multiplier of x1.17.

In a slightly more complicated example, take the Weapon Skill Seraph Blade (seraphs make excellent teachers on TP modifiers, apparently). It has the same description as Seraph Strike (Deals Light elemental damage to enemy. Damage varies with TP.), but it has one difference: at its 200% TP threshold, its damage multiplier is x2.50. This means that is you used Seraph Blade with 136% TP, your damage multiplier, as scaled proportionately to the 200% threshold, will be x1.54. However, since the 300% TP threshold is still a x3.00 multiplier, using Seraph Blade with 244% TP will result only in a multiplier of x2.72. The important implication here is that, point for point, Seraph Blade is at peak damage efficiency at 200% TP, whereas Seraph Strike is equally efficient whenever used.

Unfortunately, those two Weapon Skills are in the minority by a huge margin, as most Weapon Skills are most efficient for damage when done at 100% TP. This will apply to all Weapon Skills without "Damage varies with TP." with almost certainty, but even some Weapon Skills with "Damage varies with TP" are still most efficient when used at 100% TP, a particularly infamous example being the old level 71 Quested Weapon Skills, which were infamous for doing plenty of damage, if you bothered to wait to 300% TP to unleash them . . . but unfortunately, Savage Blade (as a typical example) was nonetheless most efficient at 100% TP anyway. "Delivers a twofold attack. Damage varies with TP." Its damage multipliers at the 100%, 200%, and 300% TP thresholds respectively are x1.00, x1.50, and x3.50 . . . but what you have to remember is that this multiplier only applies to the first hit of a Weapon Skill, all subsequent hits always being given a x1.00 damage multiplier no matter how much TP you have. So, assuming an ideal situation of all hits landing (and you do want all hits to land), you're looking at an effective x2.00, x2.50, and x4.50 damage multipliers for each threshold.

As a more cut-and-dry example, Steel Cyclone's damage multipliers were x1.50, x1.75, and x3.00 for each threshold, again, giving large returns at 300% TP, but not large enough to out-do the nonetheless underwhelming 100% version of the weapon skill three times. (One of the few of those Weapon Skills that were actually considered worthwhile was Evisceration, which instead of giving a damage multiplier at all, was a five-hit attack with a chance for a critical hit on each swing, which you don't normally get on all weapon skills.)

For your "second" point, all you're suggesting is to give a straight buff to an already-existing system. As it stands right now, yes, Skillchains do deal progressively more and more damage during each subsequent "link" in the chain; the problem (unless you're dealing with a handful of Samurai) is bothering to coordinate that many compatible Weapon Skills, which may not be the optimal Skill to perform for damage, and it usually requires waiting beyond that 100% TP threshold that most Weapon Skills are most efficient at. To be blunt, it's too much effort for too little reward, or, in the worst-case scenario, less damage than you would have accomplished just by spamming solo Weapon Skills (when the Skillchain's free additional damage fails to make up for whatever damage that may have been sacrificed in the meantime). (And even with a handful of Samurai, you're walking on eggshells; one false move and you can screw yourself out of a lot of potential damage.)

The way Skillchains work right now is that they deal a certain percentage of the damage of the Weapon Skill that caused them, this percentage increasing depending on what number link in the chain it is, and capping after the 6th link in the chain. If the Skillchain was Tier I, it will add 50% of the 2nd Weapon Skill's damage after the 2nd Weapon Skill, and if a subsequent Weapon Skill also results in a Tier I link, the Skillchain will deal an additional 10% of its respective Weapon Skill's damage, capping at 90% of the damage of a 6th+ Skillchain link resulting in a Tier I.

For Tier II Skillchains, the damage added after the 2nd Weapon Skill is 60% of its damage. If the 3rd link of a Skillchain results in a Tier II Skillchain, that Skillchain will deal 75% of the third Weapon Skill's damage. If a subsequent link results in a Tier II Skillchain, it will deal an additional 25% of its respective Weapon Skill's damage, capping at 150% of the damage of a 6th+ Skillchain link resulting in a Tier II.

If the 2nd Weapon Skill results in a Tier III Skillchain right off the bat, that Skillchain will deal 100% of the 2nd Weapon Skill's damage. If the 3rd link results in a Tier III, that Skillchain will deal 150% of the 3rd Weapon Skill's damage. If a subsequent link results in a Tier III, that Skillchain will deal an additional 25% of its respective Weapon Skill's damage for each subsequent link of the chain, capping at 225% of the damage of the 6th+ Skillchain link resulting in a Tier III.

Concerning your point on Magic Bursts, yes, even those already get more powerful the longer the Skillchain is. The bonus on Magic damage for bursting on a 2nd-link Skillchain is 30%, increasing by an additional 5% for each step later in the Skillchain that is Bursted on (presumably also capping after the 6th+ link, but this is uncertain).

Finally, concerning your complaints about the death of SC+MB and the rise of TP burns . . . hello, 2007! Meanwhile, back here in 2011, with the advent of Abyssea, we basically have an everything-burn that can give very large amounts of exp in relatively short amounts of time, to the point where exp becomes irrelevant all together for many players! If anything, the most popular "specific" burn nowadays seems to be the Fell Cleave burn, and even that is because it can be tooled for specific purposes.

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tl;dr: What you're suggesting is already in the game.