Is to have a hard cap on crit range and multiplier regardles of weapon and let some special abilities go past it temporarly/on special condition.
I'd say a +10 to crit range and a +5 to crit multiplier as a max consistant.
That way you can decide to invest feats/action points/destiny points to extend the range or multiplier or just pick up that other weapo to get to the point you want to be.
Special enhancements, like +1 multiplier on a natural roll of 19 or 20 as well as extending the critrange by +1 if you didn't crit, could still apply.
This way the existing mechanics can remain in place.