a casting version of the BAB

[TSED]

I like that it encourages multiclassing.

If I am interpreting this correctly, it basically replaces caster level, correct?

So a 10th level wizard / 10th level cleric would have a caster level of 20 for any spells it casts, but only have access to 5th level spells from either side?

That seems like a poor trade off to me.

Hmm.

If you let them cast full, then no one would ever go straight caster. EVERYONE would take as many dips in full caster classes as possible in order to get access to 5+ sets of full casting (wizard, bard maybe, druid, cleric, sorcerer, then you start getting Dread Necromancer and Favoured Soul and etc.) and absolute madness and brokenness ensues.

So that's out.

[EDIT:: Lots of discussion happened whilst I was thinking about this. I see I was interpreting that wrong, but I still like the idea I formed eventually.]

What if you had, say, CAB that didn't replace caster level, and then the straight casting classes wouldn't actually get full CAB?

Maybe the more hybridy classes would get full CAB (bard, rogue, ranger, paladin, monk?) and every one else gets 0.75 (no need to gimp melee that want to multiclass). Then these levels apply as such to other casting classes.

So a 10 / 10 wizard / cleric would cast spells as a 17th wizard and 17th cleric with a CL of 17 for each.

Hmm. Still broken, but better.

Actually, I take that back about the hybrids getting full CAB. If you do that, you can go wizard 1 / rogue 19 and voila, full wizard casting as a 19th level rogue.

Ok, what if every one has 1/2 CAB progression? A straight 10/10 split gets you access to 8th level spells and whatever goodies come from the other class, which isn't terribly broken but is generally favourable to straight classing in whatever. (Why be a ranger when you can be a druid / fighter and get more feats AND 8th level druid spells AND an animal companion AND wildshaping?)

Seems like no matter what, you really encourage taking casting multiclassing, unless they have their heart set on batman.

Could be fun, though.