Yes, a spell would be fine. I prefer a class ability, as it doesn't use up a casting slot. Of course, neither does a permanent spell, but that still uses up one of a sorcerer's known spells.
There are just so many ways to go with this!
You could do enhancements as spells (I definitely think any spell you can normally put on a missile weapon should work just fine). The write-up I did includes a note that you can enhance them just like ranged weapons, so the +3 keen flaming magestrike is fine (if a bit expensive

). I would even say in this case that you can acquire such an item, and make it into your own magestrike, while keeping the bonuses.
Or you could do enhancements as feats, in which case they automatically apply when you make a new magestrike. I think this would be appropriate for upping the die type.
You should also be able to take any normal weapon feats with it, including rapid reload.
I'd even allow Craft Magestrike to be a feat so non-casters can have them:
Craft Magestrike
Requirements: Ability to cast 1st level arcane spells OR 5 ranks in Spellcraft and 5 ranks in Use Magic Device
Benefit: You can craft a magestrike. This takes 24 hours and consumes 50 GP worth of magical materials.
Special: Wizards and Sorcerers gain Craft Magestrike as a bonus feat at first level.
Actually that answers the question of whether bards should get this ability. They can take the feat if they want to but don't get it as a bonus feat.