3 slots sounds cool. It would provide a good deal of customization without making every situation manageable by one build. I'd like to see something like:
[b]1.[/b] Primary 1 (assault rifle, sniper rifle, shotgun, smg, lmg)
[b]2.[/b] Secondary 1 (melee weapon, binoculars, pistol, other useful items)
[b]3.[/b] Primary 2/Secondary 2
While a Titan might decide to wield an assault rifle and a shotgun with only a speed penalty, it may slow down a Warlock's casting times, or make a Hunter less stealthy. You would have to weigh the pros and cons of carrying a versatile weapon and some light gear, or carrying a specialized load-out that allows you to fulfill a couple of niches while forfeiting some of your powers.