recently, i'd been involved in a number of discussions and i'd thought that i'd relate them here. essentially, they were about character concepts and themes. admittedly, D&D is a big game, but there are some areas or themes that it simply doesn't (or can't) cover. for instance, what if you're really enamoured with vampires or werewolves, but either have a DM that doesn't want a PC with that level of power, or far more likely, you simply don't want to have to eat a rather unpalatable LA that such characters can bring. sometimes however, it's merely a desire to play something from fantasy or literature that isn't exactly covered, but something similar already exists.
for example, let's say you wanted to play a mechanical-themed mage, a "tinker" or a "steampunk" character. you could try to pore through all of your books and files in hopes of finding something that is a) relative; and b) not stupidly designed or over or under-powered. you could do this, however, my suggestion would be to try a conjurer specialist wizard, except "tinker" with the spell thematics of the school, and magic. (sorry for the pun)
for instance, you could imagine - not a stretch, since the whole game is about imagining - that your wizard is a new and rare form of mage called a "tinker" because they summon semi-robotic, steampunk like constructs to do their bidding. also, during the 1 round casting time (which tinkers call "assembling"), the bits of iron, steel and wood that actually constitute the creature they're summoning whirl around and rapidly assemble themselves within moments, ready to do their creator's bidding; furthermore, he records his spells on extremely thin gears of metal, using a blocky, numerical-like script, etc. when he casts "mage armour" a slowly spinning gear phases in to guard him; when he casts "magic missile", the sound of sputtering steam-cannons belch and magically, searing bolts of iron hurl toward his foes; should he cast "identify", a rotating series of gears and magnifying glasses magically adorn his head like goggles, forever whirling and rotating, helping him see the underlying "blueprints" of other, more traditional magics; you get the idea.
mechanically, you're still a conjuror; thematically, you're something that will really impress people and stick out as new and unique. the DM really can't get too upset because you haven't really radically changed much mechanically, although he or she may get upset about the flavour of their world. realistically though, as long as you aren't intent on bringing their entire campaign down, they should be cool and let it slide. i mean, really, we can pretend that a dwarf wearing a magical corset can out-wrestle a three-story giant, wizards routinely cast spells that by their very nature radically alter the nature of the game, or that a charging paladin can, with the right spells and weapons, effectively one-shot the arch-nemesis of his deity, so really just because some new mage decides that his raison d'etre is gears, smoke and steam, people shouldn't get too upset. not that they still won't though, i mean, people are people.
another example? want to play the awesomeness of a vampire but a DM isn't keen on the powers and you're wincing over the +8 LA? i'd suggest looking to the Fiendish Codex I, as the "abyssal heritor" feats present an appopriate set of mechanics, and all that's needed to change is the name. after all, the feats cloak of the obyrith, demonic skin, eyes of the abyss, and otherworldly countenance all offer up vampire-like abilities, and have the added benefit of each gains more power, the more heritor feats you possess.
for instance, change "abyssal heritor feats" to "gravetouched soul" feats. cloak of the obyrith, which grants DR, could become ultra mortis; demonic skin, grants an AC bonus, could be called infernal protection; eyes of the abyss gives you dark vision and spot bonuses, could be called fiendish glare; finally, otherworldly countenance, which lets to "glamer" mortals like vampires do, could be deathless influence. and again, this is all simple thematics and really shouldn't upset anyone too much.