I'm not suggesting the reduced TP costs be an effect of the combos. I think best way to explain would be a few examples...

Currently if we want to do a combo with three weapon skills, assuming none of which are on cooldown, the cost to deliver this combo is 1500 TP. Now, if we remove the TP Waiving, this combo, instead, would cost upwards of 6,000TP. This simply would not work at all, so I suggest reducing the cost of each individual WS without any consideration of combos whatsoever. So, if the combo sequence in regard to TP cost looked previously like 1,500 > X > X; and each weapon skill individually cost 1,500, 1500, and 3,000... (This is a good reason why it is incredibly stupid to WS outside of combos)... these skill would then, under my suggestion, individually cost 500, 1000, 2000; and, together as a combination would total 3,500TP. The speed in which you accumulate TP would also be increased.

Now, if this were to be implemented, you could make any combination you wanted, as the limiting factor is the accumulation of TP, and the only factor to balance are the combo effects.

Just to consider the possibilities:

Current System:
You have roughly 4-6 combinations, depending on class and if you consider 1 > 2, which is a part of 1 > 2 > 3, to be a separate combo. Notice the combinations only move in one direction, so there is one limiting factor on the potential combo effects. But, 4-6 combinations would be the sum total number of possible combinations and therefore combo effects. The only way to add onto this is to implement additional weapon skills or one of my suggestions.

My Suggestion:
If you included all of my suggestions, you could have the following....
Assume you have 5 weapon skills with the following TP Costs: 1 @500TP; 2@1000TP; 3@1000TP; 4@1500TP; 5@2000TP.
These are just some of the possible combinations....

1 > 2 Costing 1,500TP

We can get creative and start assigning combo effects. Let's say that for 1 > 2, the effect is 10% increased damage for 2 and all following weapon skills that are apart of the current combo sequence.

Next, 1 > 2 > 4; Costing 3,000TP.

1 > 2 would have increased damage of both 2 & 4, and, in addition, 2 > 4 would have it's own combo effect, let's say 2 > 4 cripples the enemy (lowers attack and movement speed).

You could also have:
4 > 2 > 1; still costing 3,000TP; but having different combo effects. Instead of just getting a flat increase of damage, 2 > 1, for example could become a double attack and 4 > 2 could reduce the cooldown of 4.

I'm simply just trying to point out that, the current system is so limited and there is the possibility for MUCH MUCH more creativity and choice.