First I would look into early entry methods for mystic theurge. The class becomes much better the earlier you can get in
Go for the druid domain. Menhir Savant is a good archetype
For Wizard school I recomend taking one that starts off strong without having to scale. Admixture, Foresight, Arcane Crafter, etc.
Brew potion is only useful with potion support particularly when you have scribe scroll. Things like potion glutton and accelerated drinker would be good to take
For craft arms and armor I recommend looking at training enchant weapons(inner sea intrigue), Djezet Skin(armor masters handbook), determination armor enchant, dueling weapon enchantment(initiative bonus), and adding called to your staffs
Instead of craft construct take protector of the people. Same thing but with a nice bonus. Grow plants and craft oozes tend to be just worse versions of craft construct. I recommend looking into clockwork creatures as they have a repair drone so you never have to spend resources healing them
Spells known is typically ignored for crafting purposes as you can ignore a perquisite by increasing the spellcraft dc by 5 and spellcraft is really easy to increase. The exceptions being spell trigger items which actually require the spell
Magical Knack, spark of creation, reactionary, etc for traits
How are you starting with improved familiar? You won't have CL7 until level 10
Also there are more than 1 mystic theurge style prcs from 3.5 if your dm is willing to import
If your DM is open, Google "Pathfinder arcane hierophant" it's a druid/wizard hybrid class and is really good.
Also both mystic theurge and arcane hierophant are pretty horrible if you can't go early entry in one of them, since you have custom races, grab a lvl2 spell like ability to qualify for early entry (the RAWness of this is murky though).
Finally have you considered that you could take sorcerer and the empyrean bloodline in order to have both arcane and divine spellcasting key off wisdom?
Faith Magic. Followed by the guild membership that will let you recoup your lost level.
Alaster, most people don't wait 10 months between posting a request for advice and using it. I'm not saying you're certainly too late, but you probably are.
Hello, Bober and I wish you great games :D
To begin with, I am pretty sure that Mystic Theurge gives level to only 1 of the 2 spellcasting classes, per level not both. I don't like it but I think that's the official way.
Hi AlasterGR, welcome to the forums.
First off, as avr said, this person was asking for advice a long time ago, and likely won't see this post. You can see the time and date at the top right of each post (don't stress too much, we've all "resurrected" a thread or two in our time). Forum ettiquette is generally to start a new thread if the old one isn't active any more. If you're referencing an old thread you can create a link to their thread so that people know what you're talking about.
Secondly, i have some good news for you regarding Mystic Thurge:
When a new mystic theurge level is gained, the character gains new spells per day as if he had also gained a level in any one arcane spellcasting class he belonged to before he added the prestige class AND any one divine spellcasting class he belonged to previously.
You totally do increase both your arcane and your divine casting abilities every level with mystic theurge - that's the main draw of this prestige class.