Steve Irwin would've hated that

The statement "go awry with newer players" worries me, as a game system, any system, is explicitly for newer players. The system is, most importantly, a document to refer to for clarity and to resolve dispute, not to make things more complicated or open-ended. If players/DMs want to open it up, they can do so themselves, but my advice is to design with rules lawyers and power gamers in mind- the people who are going to 'game' the system.

By 'd20' based, I mean something quite specific- that is, using the basics of WotC's 'd20' system. That way you'll target the largest group of gamers. Definitely, definitely look at 3E. It's got a lot of great ideas, and is a step towards the skillpoint system, as the idea of 'classes' is much more fluid than 1E or 2E.

Sorry, but I don't have the time to re-design skeletons. And zombies. And wolves. And bigger wolves. That's like a 3 month project. I think TSR/WotC have done great jobs with monsters, and there's tons of books out there. Honestly, the only truly original monsters I think I've made are conjuring eyes and the crystal mage, both of which were sorta insanely overpowered. Everything else has just been a variant of stuff in core books. Some of my campaign-driving ideas have come from flipping through the monster manuals... "wait a sec, these arcanodaemons can be 18th level magic users? Illithids have 95% MR? Ixitxachitl can be vampire masters?"

I'd be more interested in designing a complete skill system, including skills for monsters.

So, here's a suggestion. Have six different kinds of skillpoints, locked to the six ability scores. PCs start with 6s, say, in each of their six stats, and get 2o-something points to raise them up initially (so ability score stats are fairly generated- this is the best way, really, as much as you love 3d6 6 times). Then, you get a number of STR skillpoints as a function of STR, etc. Meaning that fighter types and mage types don't just both begin with 200 skillpoints to buy whatever. Fighter types, generally having a higher STR, will find it easier to initially have STR type skills (combat, damage doing, armor breaking, rock climbing). And so on. We never tried it this way, but I was about to make this change before 3E came out and I threw away the skill system. My guess is that an initial bias on the skillpoints, and having different types of skills, is more optimal. Might be some allowance for using STR skills to buy other skills, such as a 2:1 exchange for CON skills, 3:1 for DEX, and 5:1 for IQ, WIS, and CHA.

Are you going to do any world design? Designing monsters makes sense only in a campaign world context I think, and it could be neat to design some analog of 'guilds' or 'prestige classes' to expand the skill system.