First off, I'm not convinced Wizards even need this ability at all...
Item Bond could have SOME type of bonus, just not a bonus Spontaneous spell.
Why not something more similar to how Familiars work?
A skill/Save bonus and Bonded Item Enchanting instead of a Familiar's capabilities? More of an equal choice, in my eyes...
(Part of my questioning of this ability is because Wizards ALREADY get a bonus spell - INCLUDING Generalist Wizards now.)
*IF* the Bonus Spell is staying in some form,
should it say "you can cast any spell PREPARED and NOT YET CAST for the day"?
Otherwise spontaneous casting anything in your spellbook (even if separated from you!?) is pretty overpowered,
and stepping on the Sorcerors' territory as well - it's WAAAY better for Wizards than Sorcerors,
if Wizards can choose anything in their spellbook (or even SCROLLS!?) as opposed to the Sorceror's limited selection.
The thing is, being able to Spontaneously Cast ONE spell/ day HUGELY appropriates the Sorceror's advantages, because realistically, you don't need to Spontaneous Cast EVERY spell, it's only necessary for those one or two you wouldn't have prepared normally. Making it apply only to spells you haven't yet cast makes it more of a choice, since you're deciding when you use it that you think THAT SPELL is one you want to cast once more that day. These changes would make it more balanced, yet still powerful & desirable.
As well, why not make it so Specialist Wizards can ONLY use it on spells from their School?
And why not implement a spell level cap, say half the highest level? This feels better than giving the Wizard a bonus Spontaneous spell at their highest level, when I don't think they REALLY need a power BOOST. As is, you don't need to advance in the CLASS, you can go into ArchMage/Mystic Theurge and apply your Arcana Bond ability to the higher Spell Levels you gain from those classes.(!)
...Overall, I think the Bonded Item's Bonus Spell should be removed and replaced with powers more on par with a Familiar,
as it seems just way to overpowered, certainly compared to a Familiar,
as well as simultaneously "replicating" both School Bonus Spells (now applying to Generalist too?)
AND out-doing and even SURPASSING the Sorceror's advantage over Wizards.
(It ISN'T such an amazing power for an Arcane Sorceror, since they know a fixed,
limited number of spells that they already spontaneously cast.)
As well, given that the spell casting progression has been stated to NOT be open to change,
why so many options (Bonded Item, Generalist Bonus Spells) which give more spell slots?
bump?