balancing...

[Jayngfet]

I'm planning on making a base class based on having a monsterous spider animal companion, It's going to be like ranger except with animal companions and spells at first level, and a spell list based on alignment

I was planning on

adding a template to the spider based on alignment, it casts all but a few basic spells, both based on your Charisma.

giving it several abilities, such as riding spiders normally too big once your spider advances to a good size

giving it d8 or d10 hit die

proficency with simple weapons and spears, medium armor proficency

and a fair amount of skill points.

balance issues?

[bbugg]

I'd stat it out more fully and then post it for comment - it's tough to really see what you're getting at with just an outline.

[dman11235]

Also I'd say this is more like a PrC: too specialized to be base.

[Jayngfet]

I may not have the books but I heard the D20 modern classes are specialized...

[dman11235]

Specialized how? Like, rogue vs barbarian specialized? Or Arachnomancer vs Anarchic Initiate? I'm betting it's the first one. Something this specialized (like, one style of combat) should be a PrC. Something like a wizard, which can do any of a number of things should be a base class, possibly going into this aracnomancer. Or a ranger, which can do a lot of things (yes, more than just TWF with scimitars or archery with a long bow) but has a set general flavor. That's actually one of the most well designed classes right there.