Maybe a "blend" pot, to vary the amount of bridge-to-neck pickup signal? I'm not sure if you could leave the pickup switch so that it selects only bridge, only neck, and then, both with the blend...or if you'd have to disconnect the switch?
Someone here could probably answer that. Anyway, you could probably get a lot of variation with a blend pot, from a biting mostly-bridge w/ a bit of neck fullness, or a mostly full neck w/ a bit of bridge bite...and anywhere in between. You really wouldn't NEED the switch, because you could turn it to either 100% bridge or 100% neck, but if there was a way to also incorporate the switch, it would be much faster to switch from bridge to neck, and then maybe even have the blend pot "preset" to a different tone combination with a quick flip of the switch to the middle.
Anyway, that would give your "both pickups" setting more versatility.
Just an idea.