This is something I proposed in this thread (which was discussing whether 3+ saves made marines too powerful), but I thought I'd share it here as a rules idea.
Now the rule is fairly straightforward, and the idea behind it is to get more value of AP4, or even just any AP weapon that is worse than your opponent's majority armour. The rule is fairly simple; if your AP value is better (lower) than the target's armour save, then they don't get to make a saving throw, if your AP is equal to their armour then they suffer a -2 penalty to the roll, and if your AP is one point worse (higher) than their save, they suffer a -1 penalty. Everything else leaves an opponent's save unaffected, as does AP-.
It's a bit of a funny one to get your head around, but I made a little table to illustrate:
Now, this may also look a bit strange at first, so it's worth remembering that the diagonal line across equal AP value would actually be 7+ for AP5 versus 5+ save, and 8+ for AP6 versus 6+ save, and these rolls are of course impossible.
What this does is allow 2+, 3+, and to a lesser extent 4+ armour saves to shrug off small-arms fire, but more lethal rounds will begin to threaten them. A side effect of the rule is also that versus 5+ and 6+ armour AP5 and 6 still ignore them as they do currently, but this makes sense due to the lightness or incomplete coverage of the armour, so 4+ saves (typically representing enclosed armour) is where you start to see the ability to shrug off damage entirely.
The result is that the system slightly devalues weapons with an AP at your armour level (no longer ignores your save) but adds value to weapons with an AP one above (worse), so you wouldn't have to shell out for as many expensive AP3 weapons to deal with marines, as a bunch AP4 firepower can be decently effective too. It also serves to distinguish between AP2 and AP1, since only AP1 can ignore all armour saves.
The idea is essentially a compromise between a full-blown armour modifier system (which could end up devaluing all armour). By only applying limited modifiers power armour still shrugs off most damage without losing any effectiveness. Of course it still isn't perfect (no bolt-on rule can be really) as costs would need to change, for lower Strength AP3 and AP2 weapons in particular, but I think it's a good rule, just a shame that it's probably too late to sneak something like this in 6th edition