Anyway, the point here is this: currently trying to use a DoT in your Signature Spells is just a bad idea. That's pretty much it. The reason for that is that there is a rule in place which says that a spell with a duration must always have a cooldown, and I'm pretty sure the cooldown must always be longer than the spell itself.
Signature Spells are in the unique position where most of them have zero cooldown along with no mana cost. Because they're meant to be spammable. A fire mage can each post just throw one fireball, the fireball's strength depending on the rank of the fire mage, but the only thing it does is burn whatever it touches. It can't explode or anything else.
Now, I'm going to show two potential Signature Spells. So far as I know neither of these are actually in use, but they are both very realistic and simple spells which one could decide to use if they're the appropriate type of mage.
First example:
Iron Dragon's Claw
Rank: B-Rank
Type: Offensive/Physical (I'd say Iron but that feels weird)
Duration: Instant
Description: the user's hand or foot of choice is turned into the claw of a dragon, forged from iron. With this claw the user can perform one attack, with enough strength that upon hit the target suffers B-Rank damage (60hp). This claw attack can also cut and slice like with a real claw. After the attack the iron claw merges back into the actual hand.
A very, extremely straightforward and spammable spell. It just enhances your attack so it actually hurts.
Second example:
Venom Devil's Strike
Rank: B-Rank
Type: Offensive/Poison
Duration: Instant/2 posts (DoT)
Description: gathering foul magic around their limb of choice the user can strike at the target with the enhanced limb, dealing 1/2B-Rank damage (30hp). Upon touch the target is also poisoned for three posts, suffering another 3/4B-Rank in damage over the duration (45hp, 15hp/post). Attacking the same target over and over will refresh the duration of the poison but the damage done each post will stay the same. This spell only allows for one attack before the gathered magic dissipates. Poisoned enemies may be hindered by the pain they experience from the poison, depending on how tough they are.
Another very typical enhancer of your strikes, only this one comes from a mage who specializes in using venom and poison instead of turning their body into iron.
Now, the interesting part: the cooldown that the first example would have following the current rules? 0. It's an instant very straightforward spell, so it doesn't need a cooldown.
The second example? 4 posts. Because it has a DoT that lasts three posts.
The first example does 60hp damage total the moment that you hit somebody. The second example does 75hp damage total. Over the course of three posts.
However the first example, requiring no cooldown because it's a very simple and instant effect, can also be used in the next round. So that leaves us with the first example making it possible to deal 120hp damage in the same two rounds that the second example can deal 60hp damage.
And the third round? The first example goes again, adding another 60hp damage so we end up with a total of 180hp. Second example adds another 15hp/post for a total of 75hp.
Fourth round. First example adds another attack, total damage is now 240hp.
The second example is still waiting for the cooldown because it's a DoT of three posts and thus has a cooldown of four posts. Total damage for second example now is 75hp, because the DoT only lasts two posts so in the third post nothing happened.
Fifth round. First example adds 60hp damage, total damage done is 300hp. Second example can now attack again because the cooldown is over, adding 30hp from the attack and 15hp from the DoT that starts ticking. So the total for the second example is...120hp
The Iron Dragon's Claw did more than twice the damage of the Venom Devil's Strike.
Now, let's assume that for some reason the Venom Devil's Strike was also given no cooldown. Maybe it just slipped past the staff or something. Let's see how they hold up then.
Iron Dragon's Claw=ID
Venom Devil's Strike=VDe
First round:
ID deals 60hp damage.
VDe deals 30hp on hit+15hp from poison=45hp.
Second round:
ID deals 60hp damage, total damage done=120hp.
VDe deals 30hp on hit, DoT of 15hp/post is still active so that's 45hp, total damage done=90hp
Third round:
ID deals 60hp damage, total damage done=180hp
VDe deals 30hp on hit, DoT of 15hp/post results in 45hp damage, total damage done=135hp
Because a DoT can only deal 1/2 their Rank's damage with the initial attack and 1/4 their Rank's damage with each round, resulting in 3/4 Rank's damage each post, the VDe starts to lag behind
Now for some reason both the ID and the VDe don't attack their target for the following two posts. The ID gains nothing because all his damage is on the instant of attack, so he stays at 180hp. The VDe gains two ticks of 15hp/post so his damage ends up at 165hp.
The ID has less damage than the VDe if they both only attack once, the same amount of damage if they both attack twice, and after that the ID has more damage and the difference keeps increasing for each consecutive round of attack.
This is but one example, but I don't think it would change much if I changed the ranks. Actually, I might as well see how it changes.
Updating ID and VDe to rank A:
ID: instantly deals 80hp damage.
VDe: instantly deals 40hp damage (look up), DoT of 60hp damage over three posts, so 20hp/post. Total damage: 100hp damage.
Let's put it through the grinder.
First round:
ID: 80hp damage, total damage done=80hp
VDe: 40hp damage, DoT of 20hp/post starts ticking, total damage done=60hp
Second round:
ID: 80hp damage, total damage done=160hp
VDe: 40hp damage, DoT of 20hp/post, total damage done=120hp.
Third round:
ID: 80hp damage, total damage done=240hp
VDe: 40hp damage, DoT of 20hp/post, total damage done=180hp
Both now don't attack for two rounds so the VDe DoT runs out. That adds another 30hp, ending the comparison at 240hp for ID and 210 for VDe.
If somebody feels like checking my math it would be appreciated, but I think I got it right.
Anyway, so on a single target the DoT, even if it has no cooldown so you can apply it again each post, loses against an Instant which just punches the target each post. The DoT is only stronger if it's a single attack and after that it just starts lagging behind.
I also calculated what would happen if we added hand-to-hand damage to each strike. The results were basically the same.
Now, for the sake of completion, let's add an example of if there are three targets instead of one, and both ID and VDe keep switching between targets.
First round:
ID:
1st target receives 80hp damage.
2nd target is unharmed.
3rd target is unharmed.
VDe:
1st target receives 40hp damage+DoT of 20hp/post for a total of 60hp.
2nd target is unharmed.
3rd target is unharmed.
Second round:
ID:
1st target 80hp damage total.
2nd target 80hp damage total.
3rd target is unharmed.
VDe:
1st target 60+20=80hp damage total.
2nd target 60hp damage total.
3rd target is unharmed.
Third round:
ID:
1st target 80hp damage total.
2nd target 80hp damage total.
3rd target 80hp damage total.
VDe:
1st target 80+20=100hp damage total.
2nd target 60+20=80hp damage total.
3rd target 60hp damage total.
Fourth round:
ID:
1st target 80+80=160hp damage total.
2nd target 80hp damage total.
3rd target 80hp damage total.
VDe:
1st target 100+60=160hp damage total.
2nd target 80+20=100hp damage total.
3rd target 60+20=80hp damage total.
So...we learn that the VDe has an advantage when fighting multiple enemies.
However that advantage isn't an overwhelming one which completely puts the ID out of the competition.
More importantly, it doesn't remove the fact that against a single target the ID won the competition.
Realize that if the VDe had to follow the current rules about cooldowns he would have lost the competition both against a single target and multiple targets. Actually, he just wouldn't compete. He'd be trampled and forgotten.
Now I would ask of people to explain why the VDe would require a cooldown. Because so far as I can see just because it is a DoT doesn't mean it's automatically so strong that it requires the cooldown to keep it balanced. In fact the cooldown would immediately cripple it to the extent of being worthless.
And yes, I do realize that this is only one example of a Signature Spell with a duration. However it does look like an example which immediately questions the rule that a Signature Spell with a duration requires a cooldown. And if this one does it, who's to say other examples wouldn't?
If the only argument is 'because it's in the rules', I'll have to invoke the piece of wisdom which goes as follows: 'rules have no merit in their own right. They're tools.'
Of course now I'm really hoping I got my math right otherwise I'll look rather silly.