This is a really fun idea, and a DM of mine just ran a one-off to this effect as well. It was a blast. So having just completed this thought exercise myself, I will tell you an Expert can get pretty darn close to PC effectiveness.
Not sure if this is something to watch out for, or a mercy tip to hand out to help them along.
So you end up with bit of a hedge wizard build focusing on range, traps, and trained animals in combat, pulling off the occasional scroll or wand casting when you really need it. Otherwise you are the primary face man and occasional scout with a ton of languages known.
There are definitely two flavors of BCD spells, really nice utility call outs exclusive to other lists, and those that overlap with the Wizard spell list such that there is little need to spend time and energy scribing Wizard versions into your spell book.
You can safely avoid some relatively common spell picks during Wizard spell selection either because they are low enough the increased casting times are not prohibitive or because they make great scroll scribing targets. Though do note that you will need a strong Use Magic Device skill to reliably activate them in a pinch as they are not technically Wizard spells.
Note: * = special call out for being lower level than the equivalent Wizard version.
Level 1:
Curtesy Bard:
Charm Person
Comprehend Languages
Disguise Self
Expeditious Retreat
Grease
Heightened Awareness
Unseen Servant
Curtesy Cleric:
Air Bubble
Ant Haul
Endure Elements
Infernal Healing
Obscuring Mist
Protection from [Insert Here]
Curtesy Druid:
Hydraulic Push
Snowball
Level 2:
Curtesy Bard:
Alter Self
Detect Thoughts
Invisibility
Tongues*
Curtesy Cleric:
Bear's/Bull's/Eagle's/Fox's/Owl's
Darkness
Find Traps
Resist Energy
Shatter
Curtesy Druid:
Flaming Sphere
Fog Cloud
Frost Fall
Gust of Wind
Spider Climb
Level 3:
Curtesy Bard:
Adjustable Disguise
Clairaudience/Clairvoyance
Gaseous Form
Haste
Invisibility Sphere
Phantom Steed
Scrying*
See Invisibility
Seek Thoughts
Tiny Hut
Curtesy Cleric:
Deeper Darkness
Dispel Magic
Locate Object
Magic Circle against [Insert Here]
Curtesy Druid:
Animal Aspect, Greater
Create Treasure Map
Daylight
Protection from Energy
Sleet Storm
Water Breathing
Starting from level 4 onward your mileage will really begin to vary, and scroll costs and casting time increases (now four full rounds) begin to grow prohibitive. That said, it is worth noting some common spell progressions you can cheat your way into without taking Wizard spells:
Planar Ambassador - ie., binding, conversing, trapping, extra dimensional beings. You can BCD for Magic Circle X, Planar Ally, Planar Binding, Dimensional Anchor, Plane Shift, Astral Projection, as well as get access to excellent Cleric divinations for free.
Divination/Teleportation specialist - ie., scry-and-die, ask the DM, recon. The Cleric and Druid spell lists in particular tend to be equal, if not stronger, in these areas. Bards get Scrying at level 3, and Druid Scrying is much cheaper. Tree Stride and Transport via Plants are great utility transport spells. Ancestral Memory, Augury, Divination and the like are just better than Wizard equivalents. The only thing you can't really take advantage of are emergency retreats spells - Dimension Door, Getaway, etc. due to casting time limitations.
Base builder - Create Demiplane, Fairy Ring Retreat, Plane Shift, Wish, Miracle are all free picks for you from BCD, and at lower levels you have Web Shelter, Tiny Hut and the outstanding Hide Campsite spell from Druid level 3.
Doug M. - certainly share your pain re: spell selection. I don't think too many people would hold it against you to lean on something universally well-respected (e.g. Treantmonk's wizard guide) and have your section be more of an addendum rather than trying to recreate the wheel. Basically note any spells that change rating for the Spell Sage, and any must-picks from source material not covered in the original guide.
One more observation - and right up front this is technically sub-optimized but could really shine in campaigns, or with DM's, that are especially focused on survival-type gameplay and/or resource constraints (e.g., money, time, town visits, spell components, etc.).
The Spell Sage has the capacity to be the most resilient and versatile caster when cornered - ie., trapped in prison, spellbook stolen, in a magic-drained area, etc.
Spell Mastery as a bonus feat mid-level should give you 5-6 spells to prepare without referencing your spellbook. Pick up Eschew Materials. Grab Blood Money as one of your Spell Mastery selections, and pick your most versatile spells for 5/4/3/2 so that you can prep at least something each level.
That gives you a Spell Sage whose worst-case scenario is the ability to cast a huge number of Sorc/Wiz Conjurations and Evocations, and all 1-5 Bard/Druid/Cleric spells.
Hi Doug - great idea thanks so much! I've had so much fun reading through all the tips and tricks as I prepare my own Spell Sage for a mid-level campaign.
Two little Spell combos to consider for your next section:
Contingency + Reincarnate gives you a relatively easy and early path to immortality - not super useful in most campaigns but compelling from a role-playing perspective.
More practically, Spellcasting Contract and its big brother from the Cleric spell list are very interesting options to 1) make your Fighter love you, 2) increase efficiency in combat, 3) pull off some great round one spell combos, 4) increase your own survivability via profane bonus to saving throws, AC, checks.
I really like gifting Mirror Image and Enlarge Person to the Fighter, Shocking Grasp and Invisibility to the Rogue, etc.