The Power Modifier Concept

[Shurrikhan]

I'd rather not see this used for every skill in the game. At that point you have no balance (against GCD rotations) or prioritization in its use apart from whatever "cost" it adds. (If MP/TP, I could see this working for BLM, but being lackluster or just outright annoying for everyone else, and mechanics-breaking if usable by healers, while probably just annoying when used as a consistent crutch.)

I see a charge system as being useful for solving a number of things, such as:
- Button Bloat (Thunder I, II, and III could all just be different charge lengths)
- The Grit/Oath annoyance (not a copy of WAR's ability stance swaps, but of variable length as to be swappable in as little as 60% of a GCD, much like Meditation)
- A few particular abilities whose jobs have unique need or synergy for/with the timing and efficiency a charge system could give (e.g. Clemency, if it were a serious heal; allowing PLD a fair bit of control despite having only one real heal)

I welcome the idea of a Charge system and could similarly see Channels being used, but a charge system for every attack would disrupt almost all the intricacies of pacing we have right now. I'm not saying that ultimately the product wouldn't somehow be better, but until I see some illustration (not literal pictures, but numerous rotational descriptions) as to how it would pan out in play, I'd be against the idea.

...
As not to be purely critical though, let me toss out a thought of my own. You mentioned "this mechanic is based off the concept of momentum", but I don't really sense any momentum in something that stops and holds to couch its punches. Quite the opposite. What would feel like momentum to me, though, might be a sort of retake on combo system. This could be as simple as different combos having different effects, some having a reduced recast specifically for the next in their combo chain while others just add the normal potency and/or additional effect. Or it could even replace the primary function of combos by causing combos to create momentum, with whatever bonuses that involves, while the separate combo abilities are now better able to stand on their own otherwise.

That idea's not something that, by itself, I'd want to consider introducing into the game as is, either, but it is certainly food for thought.

[Sandpark]

The whole point is to not have prioritization in it's use. It adds a level of performance choice rather than just stat number solely. I don't consider it annoying, it's a guaranteed critical hit for users with an accuracy vs power risk reward. When you play any good sports videogame, the actual tactile pressing of keys determines the accuracy of the shot. One point I want to make is this is not a charge system, a charge system has a guaranteed power advantage by holding it down longer. Just as a guitarist can choose to hold a note longer or lightly and briskly play notes for speed alterations, so to this concept works.

A tier selector like you mentioned would work too, however that wouldn't change the dynamics of how a skill is cast only that you cast different skills, basically a charge button.

It is based off momentum.
If I swing a bat fast or tap a light swing I might be more accurate hitting a fast ball but it would not travel as far.
If I use a bit more strength, it would be a little less accurate but travel further.
If I use all my strength, I would be way less accurate but when I hit the ball it will travel max possible distance.

Now the meta is cast your heart out but you don't really have any control over how your skills are cast minus stats and gear, you cast the same way as X other <insertclasshere>. This adds a dynamic with momentum where as in you might all draw from the same skill pools, but the cast style is determined by the user.

[Shurrikhan]

The whole point is to not have prioritization in it's use. It adds a level of performance choice rather than just stat number solely. I don't consider it annoying, it's a guaranteed critical hit for users with an accuracy vs power risk reward. When you play any good sports videogame, the actual tactile pressing of keys determines the accuracy of the shot. One point I want to make is this is not a charge system, a charge system has a guaranteed power advantage by holding it down longer. Just as a guitarist can choose to hold a note longer or lightly and briskly play notes for speed alterations, so to this concept works.

A tier selector like you mentioned would work too, however that wouldn't change the dynamics of how a skill is cast only that you cast different skills, basically a charge button.

It is based off momentum.
If I swing a bat fast or tap a light swing I might be more accurate hitting a fast ball but it would not travel as far.
If I use a bit more strength, it would be a little less accurate but travel further.
If I use all my strength, I would be way less accurate but when I hit the ball it will travel max possible distance.

Now the meta is cast your heart out but you don't really have any control over how your skills are cast minus stats and gear, you cast the same way as X other <insertclasshere>. This adds a dynamic with momentum where as in you might all draw from the same skill pools, but the cast style is determined by the user.

Regardless of whether individual skills or events are intentionally prioritized via interaction with other systems or if every skill gets equal treatment and access to the power modifier mechanic, both will end up playing in an particular optimal fashion unless you intentionally set up each jobs' skills to create multiple rotations of equal viability (albeit where viability may include a lower theoretical output at a lower skill floor and options in line with such balances). The prior just tends to require more thought and adaptation. The other follows very simple rules, such as maximizing windows. Given that you're using a flat, rather than percentile, charge time, especially, it already will favor particular abilities (longer casts, on whose original cast time it has a smaller proportionate effect, and stronger abilities in general on which less rotational damage over time is wasted), and will be nerfed on speed-based classes. If your Dragon Kick / Twin Snakes will either run 2 seconds late or 3 seconds early, one can and will now make up for that by adding 1.5 seconds each to his ToD or True Strike and Demolish, perfecting his DK's or TS's uptime. Why? Because you're only increasing potency. Therefore it should only be used on the skills with maximum potency. Anywhere else, especially in the case of a Monk running at a 2.15 GCD or less, will lose some 75% of a GCD to extend a skill's damage by 50%. If not for the per-execute range of a Monk's skills, that would always be a loss.

Charging up a Twin Snakes or Dragon Kick would give you 70 or 75 extra potency at the cost of 75% of, say, a 145+ modified skill-potency per second rotation (here including DK/TS effects on AAs and oGCDs, but otherwise only weaponskills). Give or take its aid or harm in perfecting windows, that's still almost certainly going to be a loss, and absolutely will be a loss compared to alternative uses for that time. But 50% of a Demolish is 210 bonus potency. Unless it really thrashes a lucrative window, that will be worth it.

And then there's the matter of Accuracy. At 1.5 seconds to charge 50% additional damage, that may already be costly enough. It's not scaling with the actual cast, after all, so that it's worth only 85 potency on a Fire, but worth 150 on a Fire IV, for the same time spent. Moreover, what you're asking for is that people now stat into something otherwise unnecessary, bonus Accuracy, for the mere opportunity to use a new option reliably.

Consider also: you can't pre-charge, (i.e. charge without a target for the skill), so you can't use this to instantly nail an add with a 150% damage attack the moment it spawns. You also can't get an extra 50% of a GCD, effectively, into a given CD window (Raging Strikes, etc.). Granted, if one could do those things they would be obligatory unless balanced otherwise or more in conflict with weaponskill or Spell resource/buff/vuln windows than they could produce in bonus damage.