That is a solid idea, I could really get behind it. I think if all three weapon slots were spells, it might break the Warlock's ability to deal damage compared to the current weapons. I suppose you could have spells that deal analogous damage, rate of fire and range to each weapon type. Hand cannons become magic missiles, auto rifles become chain lightning, etc. Not sure how you'd introduce variability and perks in to the spells. Maybe have different schools of magic or different factions have their own spin on each type of spell. Alternately, you could make runes go only in the heavy slot. This justifies the big dramatic feel you want from a spell and puts them on the same damage scale and cool down cycle as a heavy weapon. It also makes the attack less common, which I think you'd want.