I am surprised to hear that 6V is not intense enough for you. What kind of atomizer and e-liquid are you using?
Typically 5V should be strong enough to deliver an effective vape from most regular atomizers. 6V and especially 7.4V really require an HV (i.e., high-resistance) atomizer. Even 3.7V with an LR (low-resistance) atomizer can deliver a sweet-spot experience. [In addition, the same brand atomizers with the same published resistance vary. For example, the most recent Joye 510 atty I received is not nearly as effective as one I've been using for months. You'd think it would be the other way around.]
Also some mods are more effective in utilizing the voltage from the batteries they contain. [For example, within the world of 3.7V, my Provape-1 delivers a more intense hit than my DSE905, even though they both use the 14500 battery.]
And, as I mentioned at the top, not all e-liquids (even at the same nicotine level) are created equal. Experiment to see which you like best.
I do not think we will be seeing even higher voltage mods coming out. 7.4V already is plenty strong enough to burn out atomizers.