To start, let's clarify a few things. I am currently using Labyrinth Lord and the LLAEC as the basis. I will also be altering rules and dove-tailing other rules from other sources as I see fit. Shocking, I know, because that is never done. ;)
So here is a quick list of what is in, what is out and what is in but with a twist or certain reservations at this point…
Racial Classes & Subraces with classes - both IN, so you can be an Elf or an Elven Ranger. Not sure what I want to do about racial limits yet. I've never liked them but don't know if I want to spend the time thinking up a balanced way to replace/eliminate them. Also, I think I will have variant builds for each class (perhaps the only thing I like from 4e). Probably a campaign specific variant (ex. Piecemeal Warrior [fighter variant]) and a race specific variant, for a race that breaks the standard mold (ex. Fetchling Witch [cleric variant]).
Assassins - OUT, will probably be a thief or monk variant.
Bards - MAYBE, but only if I can guarantee that they turn out like this:
Clerics - IN
Druids - OUT, kind of. Fetchling Witches are nature-clerics.
Dwarves - IN, standard dwarves just "showed up" at some point and no one knows where from.
Elves - IN, BUT elves are the descendants of human-martian hybrids (the result of genetic experiments not racial interbreeding). During the Martian Wars the alien invaders began experimenting with human-martian hybrids as a way to assimilate the best of both races. Now, many hundreds to thousands years hence these "elves" have formed their own culture.
Fetchlings - IN, surviving "Martian Pod People" evolve into their own species with shape-shifting abilities.
Gnomes - OUT
Goblins - IN, BUT they are the direct descendants of the martian invaders.
Half-Elves - OUT
Half-Orcs - IN, BUT they are simply called Orcs and are the result of human-elf interbreeding, instead of half-elves.
Humans - IN, with a "mutant" class option so humans have a racial class, too.
Fighters - IN
Halflings - IN, BUT are not hobbit-like. Halflings are literally shrunken humans. At some point in the distant past as humans (and martians) went underground to escape the fallout a small enclave led by a scientific genius shrunk themselves to half their normal size to avoid overcrowding in their underground habitat. This isolated group form what is now the Halfling race.
Illusionists - MAYBE, possibly psionic (see Mystics below).
Magic-Users - IN, BUT I'm not sure how I want to handle them yet. I want more of a magic as super-science feel but have not figured out how to do it.
Monks - IN, BUT more of a pugilist or improvised weapon expert than a martial artist.
Myconids - IN, to paraphrase "God, shmod, I want my mushroom men."
Mystics - MAYBE, I am toying with the idea of psionics.
Paladins - OUT, if I kept them I would make them more of a knight class and for that you can just make a Fighter, or even a Ranger, with a code.
Rangers - IN, BUT no magic. More of a scout/archer.
Thieves - IN