Anobrega has four groups of people who I see as having spellcasters: Calthi (Celtic analogues), Toreans (Roman analogues), Maradani (Roma analogues), and Elves. One thing I'd like to see, is that each grouping is more or less unique.
The Calthi are probably the most straightforward: they have druids. I don't think a second spellcasting class is necessary for them, and as it stands with them I'm leaning toward the druid class as written in Eldritch Wizardry. The other no-brainer for me are the Toreans - they have an order of Lawful clerics devoted to Deus Sol Invictus (the unconquered sun), the sun god in a militant guise. The cult itself is almost wholly military, based on the one fostered by Aurelian in particular, one of the best of the barracks emperors, and lends itself well as the sort of "templar" character embodied in the cleric. There are plenty of other priests - in Torean society as in pagan Rome being a "priest" is an honor conferred on some noble or other for ceremonial reasons - but a militant Lawful cleric is a member of this cult. Chaotic clerics are secret priests of Bacchus and have to keep their class under cover, quite different from the open military priests of Sol Invictus.
All that covers the religious casters, and I think the D&D standard is pretty good for my purposes. But then there are magic-users, and I'm a bit torn. Celtic magic is druidism, and I see no need to have Calthi magic users. Roman magic is a bit of a mixed bag, quite a lot of it had to do with divination (through all sorts of methods), while many reputed magicians had abilities that map better to clerics - miracles and the like. It was also low-status, definitely below religion in terms of overall prestige. The D&D magic-user is not a close match for this.
One thing I've been looking at is the Pyrologist, a class that Len Lakofka claimed in his fanzine was Gygaxian but turns out to have been his own work. I think the balance might be slightly off from the default magic-user, but there are two things that really draw me to this class. One, it comes from an old school APA-zine, and by one of the authors who contributed a lot to the AD&D era. Two, the elemental connection is thematically something I'm very interested in. The classical elements were more Greek but fit in well as a way that Torean magic could be something other than "stock D&D magic." I think it's more of a good jumping off point for a set of old school elementalists, though, each having its own replacement for the stock D&D m-u spell list, and also with its own unique not-quite-a-spell power (fire and air are light and wind, water is going to be purifying water, and earth is doing...something...).
Rounding out humans is Maradani, who are a great category to have witches, although this might wind up being an NPC only type using the classic "witch" NPC class, unless somebody can offer me a witch class that really would work well as a PC type. (The one I'm thinking of is from Best of the Dragon #1 and not quite so good for my purposes.) Witchcraft and curses are the direction to go here in general.
All of which leaves me with two classic D&D elements: the magic-user and the elf. I am seriously tempted to just shove the two archetypes together and say that a standard, no-frills magic-user is also a standard, no-frills elf, toss out level limits and the whole dual class / multi class / race as class concept for them. Elves will be either fighters or magic-users, single class. It's not a perfect match but there are things that can make it work. I'm thinking that the reason elves have the "default" magic is not so much that it takes certain races to do it, but that the physical process of learning magic takes so long that no other race lives long enough.
As usual I'm more than open to ideas, opinions, denunciations, and so forth.