Hmm, nice ideas but .....
1. Is keen allowed on a bow (I thought it only works on arrows). I hope I'm wrong 'cause a keen crossbow would be nice for my mage.
2. If you take frost, flaming, thundering, xxx, yyy, zzz ... How many standard actions would it take to activate all those nice gimmics ? One for all together or one for every flaming, shocking, acidic, frost and speed ?
Oh, and never underestimate a natural +5 bow. No need for the cleric to waste a spell, no dispelling, runs 24 hours a day. Slightly harder to sunder

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Hm, what about one or two elemental damages like sonicburst (don't know the ingame name of it) and flaming burst. Since a bow has a x3 crit it would do some nasty extra damage on a crit. Something like 2W8+1W6sonic+1W6fire+2W10Sonic+2W10fire+10enhancement sounds really critical to me. Remember: normal elemental damage isn't multiplied at critical hits.
Take Improved crit (Comp. longbow) and a level of deepwoodsniper (all arrows are keen) to get a crit from 18 to 20.
Just my 2cents.
Bye