Here's what I've got for the Medium in my house rules. I really like the flavor of this class and I think it can be good, but it needs some work. Thoughts?

Spellcasting

Spell casting progression like the bard/mesmerist/spiritualist. Use spiritualist spell list for lvl 5 and 6. If a spell shows up on both the Medium and the spiritualist spell list at different levels, you get it at the earlier level.

If the Medium's other abilities were comparable to the bard's spell progression their spell progression would be okay, but they simply don't.

Spirit Surge

The dice size increases to 1d8 at lvl 5, 1d10 at level 10, 2d6 at level 15, and 2d8 at level 20.

An extra 1d6 doesn't cut it when you get to mid-levels and above. The escalation of the dice matches the Warpriest's sacred weapon damage dice.

Favored Locations

Favored locations are not required to channel the spirit you want. However, if you do channel a spirit in their Favored Location, the Spirit Bonus increases by 1.

I like the idea of favored locations, but not the requirement for them. Sure, most DM's will be loose with this requirement, but that's not really a solution to the problem.

Archmage Arcana

Spell Focus of your choice for free while channeling the Archmage. The type of Spell Focus you have can be different every time you channel the Archmage. At level 10, you also get Greater Spell Focus in the same school of magic.

Also, for each level of spell you can now cast (including level 0), each time you channel an archmage spirit, select a single spell of that level from the sorcerer/wizard spell list to add to your medium spell list and spells known until you lose contact with the archmage. When you cast these spells, they count as arcane (though not for the purpose of fulfilling prerequisites), and thus you must provide verbal and somatic components instead of thought and emotion components.

Changed this, since the Medium now has the bard's spell progression.

Divine Surge

You can touch yourself or an ally as a move action to relieve the following conditions: fatigued, shaken, or sickened. At level 5 you can also relieve dazed, diseased, or staggered. At level 10 you can relieve cursed, exhausted, frightened, nauseated, or poisoned. At level 15 you can relieve blindness, deafness, paralysis, or stunned. You can do this an amount of times per day equal to your Charisma modifier.

Also, for each level of spell you can now cast (including level 0), each time you channel a hierophant spirit, select a single spell of that level from the cleric/oracle spell list to add to your medium spell list and spells known until you lose contact with the archmage. When you cast these spells, they count as divine (though not for the purpose of fulfilling prerequisites), and thus you need a divine focus if the spell requires one.

The first part is similar to the paladin's mercies. I feel like being in a position to relieve these conditions is uncommon enough to be on par with a well chosen Spell Focus, though I'm not 100% sure.