Psikerlord#
Explorer
For expanding the crit ranges, other than the moonblade, the closest things that I've found are the Fighter (Champion)'s Improved and Superior Critical abilities. This lends itself to the idea that the system can support threat ranges of 18-20, but it's so powerful, it ought to be really rare.
Yeah I would suggest if you are going to introduce a keen weapon, I would use +0 bonus, similar to the flame tongue, and I'd make 19 or 20 crits only.
I would definintely not introduce such an item in a game where a PC is a chamption fighter however - you are would be giving away his best ability for free!