First off there is already a great bow (It's an exotic bow found on pages 154 & 156 of Complete Warrior, with the composite versions on the same pages)
Composite Greatbow
Cost: 200 GP
Damage (M): 1d10
Critical: 20/x3
Range: 130 ft
Weight: 6 lbs
Type: Piercing
I know, i actually went and looked at it, which is why my description and given price matches it, well except for the "Great Bow" line, anyways.
Versatile Archer [General]
You have trained in unusual forms of archery.
Prerequisites: Point Blank Shot, Mounted Archery, BAB 3+
Benefit: When using bows normally restricted from use while mounted, you may use them while mounted.
Special: A Fighter may select Versatile Archer as one of his Fighter Bonus Feats
this one's new i'll have to look at it, and chances are apply it to my character if i can.
For the Greatbow, the Eldritch version isn't going to be as useful for quite a while with the setting.
i thought as much, however, having something like that can be handy at the oddest of moments IMHO, in fact personally the only downside is not working in Antimagic Zones, if it could work but not come out again until you leave the Zone, if it's "destroyed" would you allow that?
I mean seriously i can have it altered to an increased pull if i get Str bonus's later on, if i get hit with a permanent -2 to Str i can have the pull reduced so i'm not taking an additional -2 penalty, i can have it enchanted to produce arrows on it's own, etc. it's pretty much the perfect "versatile" weapon even if i've currently got something better, not to mention i don't need a move action to pull it out, Free Action as long as my hand isn't full, which is another reason i was going for Claw Weapons and Unarmed Fighting, something like this simply complements it even more if someone gets in close.
edit: the last line didn't quite come out right, in other words with my Bladed Gauntlet I've got both a Ranged Attack and a non-"Attack of Opportunity" Melee attack ready if i need it. I was originally going to simply go for having a melee option in HtH with Bows being the Specialist until this idea came to me after you mentioned upgrading things into magic items.