DraQ:
Yeah, it's interesting. I've been thinking a lot about (ab)using the multiclassing system in Wiz8 and figured that specialist spellcasters are the best for converting into hybrids (at lvl5) or elites.
With specialist-bishop conversion it's pretty much a no-brainer. You get to keep whatever skills you learned, get access to other spellbooks and better equipment selection at the cost of the higher levelling cost for elite classes. IMHO the most profitable one to do is mage->bishop, as you get to keep the starting mage bonus to wizardry and 5 skill points in all of the elemental domains, which - needless to say - is a huge help early game in making your bishop carry his own weight.
At an added side, specialists have lower initial stat requirements which means that even when you add the required stats to be able to multiclass into a bishop you can put more points into intelligence and piety from the get go.
A short sidestep towards bishops here - many people on the net claim that a 4-school bishop is the way to go. It simply is not, unless he's the only spellcaster in your party or you love grinding and gimping yourself. First you need to brave the Arnika road with a dead weight instead of an able caster. Then you need to grind alchemy to sell pots to get the cash for spellbooks. Then you need to practice a lot to make the bishop barely able to cast anything at all, because since you weren't really casting all the way to Arnika pretty much only SP he's got in the relevant skills are from levelups and his domains are probably empty.
It's much better to focus your bishop on two schools from the get go. This allows you to be more liberal with spellpicks, cast away in the monastery etc. In Arnika, if you feel like it, you can buy some spells from neglected schools and grind him up anyway but I don't think it's worth it. In other words - having two spellbooks is a large advantage over having just one. Having four spellbooks is a meagre advantage over having the two of them.
The specialist-hybrid conversion needs to be tested. I was thinking about making a dracon priest and converting her into a valkyrie at level 5. That way she retains her divinity skill remaining a competent healer which allows her to concentrate on improving fighting skills. On one hand this is the better way if you want a competent spellcasting hybrid; on the other I don't know what happens with spell selection at that point. By lvl5 an ordinary priest would have access to level 3 spells while a Hybrid would need lvl 9 for that; the question is if you lose the lvl2 spells you already had. Still, seems a better deal than sticking with the priest which seems to be a waste - if you want healing you can get an alchemist, if you want healing&buffs get a divinity-focused bishop.
I'm also considering dualing the priest into a lord at lvl5. That way he'd also have a bit of mace training to kickstart his fighting.
Let's philosoraptor a little about multiclassing in general:
FIGHTER: IDK if it makes much sense to start with a fighter and multiclass into something else. You could do it for better initial stat distribution, I suppose. Say, start out with a Felpurr fighter, get your speed up to 70 that way. You'd then be able to multiclass into a samurai at lvl 3 (need 5 more points spent on INT) and AFAIK there's no downside to this (aside from the samurai starting equipment which isn't that stellar) so it boils down to minmaxing. It would be a decent deal if you planned a non-spellcasting samurai. Different variations would be possible with other hybrids.
HYBRIDS: If you start out with these I suppose it's simply better to stick to them without multiclassing. I can't see any obvious benefits of changing classes; hybrids are cool due to their class-specific abilities (cheat death, lightning stike etc) and you lose them when you switch.
BARD&GADGETEER: The gadgeteer is out of the question UNLESS you are willing to abandon the omnigun. If so, a switch to modern-weapons based ranger is possible when you max out engineering; it will be so late in the game that there's the question whether it's worth it at all though. Same with the bard though I suppose it's easier to get music up than engineering. Due to these classes dependence on skills they're also a poor target to switch from something else.
ROGUE: Can be used for better stat distribution just like fighters and specialist mages - use it as a background for the monk, the ninja and the ranger. Also, a tempting idea is to get stealth up and then switch to fighter; I suppose the AC you'd reach with heavy armor and stealth could get ridiculous, and berserk can substitute for lost backstabbing ability. You also can build up swords instead of daggers so that when you switch to fighter you get a dual-wielding unhittable motherfucker. Switching a rogue into a samurai could work too.
SPECIALIST CASTERS: described above, with the caveat that the only real advantage specialist casters hold over bishops seems to be the quicker levelling. Priest's special is too random, Alchemist's special becomes useless once you get access to decent vendors, psionic's and mage's specials are all-round meh as you'll be casting shielding spells anyway. The bonus to relevant schools probably stops being so relevant around mid-game unless you're a masochist and chose a four school bishop in which case you can masturbate to having a useful character near the end-game or like the tedium of grinding charm, mindread and knock-knock.
The thing is - you can multiclass to a bishop anytime you want. A lvl2 multiclass makes a better bishop already but if you want to specialise further in a single school then you can wait a couple more levels before bishopification. Switching at lvl5 will make your primary spellbook well developed, and the skill points you've gathered in magic domains will help you learn other books quickly. I'd go as far as to say that a multiclassed bishop is pretty much always better than a vanilla bishop, even if you go for a 4-book one due to the fact that when you switch you can actually cast shit.
Another thing, also described above, is switching a spec caster into a hybrid. A wizard-> samurai or psionic->monk don't sound that good (both wiz and psi are exceedingly squishy while sam and monk are supposed to be out front), but an alchemist->ranger is better; the priest seems the best choice for hybridisation as the divinity spellbook is rather lacklustre on its own; switching him to a lord or a valk (keep in mind that you'll start with 0 polearms though) is the quickest way to get a competent fighter/healer/buffer.
An alchemist->ranger or priest->valk could be a decent 4th character, with their ranged/extended attacks and spellcasting. A priest->lord could make a decent frontliner with better spellcasting then your vanilla lord would probably achieve. An alchemist-> ninja is also a possibility since your DEX is high and you'd use thrown weapons from the start, but keep in mind that if you switch to a ninja you won't get the ninja armor which means that you'll be running around in robes till the end of the game.