I'm a *long* time player and recently rediscovered the game. I'm planning on doing a full play through (BG, SOD, BG2) with a character.
I've got my Assassin through to Chapter 7 of BG and then got to thinking, wouldn't it be great if he could do *some* magic as well? A few spells here and there to support his thieving skills.
I don't like the idea of a multi-class Thief / Mage which plays the two parts equally, so wondered about doing a Human dual class Mage -> Thief (and then shamelessly cheat and give him the Assassin kit). I think this works from a role-play perspective - the studious mage is thrown into the big, bad world, his arcane capabilities aren't enough so becomes a "thief" (essentially the opposite direction to Imoen, but with a VERY different skill set).
I suspect this won't be too powerful - the types of spells I'd want are largely non-combat (I can think of Knock, Friends, Identify etc) and to be used only when safe to take armour off, although does give early access to scrolls and wands. By being an Assassin rather than general thief the advancement in skills is much slower, but obviously more capable in combat (and poisoning enemy mages is lots of fun).
I'm guessing that dual-classing quite early (level 3 or 4, giving him access to second level spells?) means he doesn't get too many spells and will quickly level back up with the low XP requirements for the Thief class. Any later and he's probably too invested in the magic path and will be starting to become more powerful.
I would have considered starting out as a specialist mage (probably an Illusionist, which kind of supports the character idea), but whilst adding a kit to a dual character is possible I don't think two kits would work.
Aside from CHARNAME, I'm likely to use a fairly canonical party for the majority of the game with the occasional switch out to do specific quests.
Anything I'm forgetting, or any comments, especially about how this might play out in the later games and at higher levels?