RE: New Feats
I understand that there is a glut in the amount of new feats (and also spells and creatures) out there. However, what I believe is missing is a quantity of useful, entertaining, and above all playtested feats (and spells and creatures). Every feat that we have created (and there's a lot, because feats are probably the easiest thing to create) fills a gap. One of the more interesting categories of feats (In my opinion) that hasn't existed yet is feats with an alignment requirement, for those characters who are truly dedicated to an ideal.
Additionally, metamagic feats are ample, but most outside of the player's handbook seem to be over-powered or underpowered. With the sheer scope of magic, there will always be room for more metamagic feats and feats involving magic (such as Monte Cooke's Eldritch feats).
RE: Campaign Settings
I really wish I had the time to do one justice, but I certainly don't want my debut product to be a campaign setting, as there just isn't time to make one worthwhile right now.
RE: New Spells:
Much like feats, there is a glut of spells available out there. However, there is always room for new and interesting ideas. Also, playtesting returns as a concern. Although a large number of products have well-designed and intersting spells in them, they almost all also seem to have some over-powered or under-powered items slip through.
RE: Prestige Classes
The next largest category of creations we've got is Prestige classes. That's because, to me at least, they're the most fun to create. I enjoy figuring out an idea for a character, group, or whatever and then molding it into 5 or 10 levels of Prestige Class.
Some of the items we've created are the True Artist, who meshes magic with painting to crete astounding effects; the Cabalist, a member of a magical group who excels at ritual magic within his group; the Chosen, Champion of their Deity, much like the Dragon class in Tome and Blood, but their prerequisites and benefits depend upon what deity they follow.
RE: Core Classes
Core classes can be much harder to create. For starters, the existing ones seem to fill every gap, at least on the surface. However, there are holes. For instance, what if I want to pla a divine caster that does not have to memorize spells? Or a decent Martial Artist that isn't a lawful member of a monastic group?
Above all with core classes, playtesting becomes extremely difficult, as you've got 20 levels to contend with, and so many other core classes to compare them to. If a prestige class becomes too powerful, its fairly easy to tack on more prerequisites and / or remove or curtail abilities. You lose the prerequisites fix with core classes.
RE: New Rules
The only new rules we have in the works right now are Ritual Casting and Mana rules. Although Relics and Rituals did a fairly good job, the system is a bit sloppy in places, and allow too much leeway in some areas and not enough in others.
The mana rules are what the Mana Weaving skill is based on, and one of the most interesting things they provide for is a way to have two spellcasters duel without having to lob fireballs in the city streets.
RE: Variant Rules
Although not currently in document format, many of my house rules could be codified and added to a book. My group uses many variants on the rules (such as different versions of over-powered spells, some different combat modifiers, etc.). I doubt the interest would be too great in them, but I figured I should mention them.
RE: New Magic Items
New magic items are quite possibly one of the easiest thing to create, as the rules are mostly right there in the DMG. However, playtesting becomes a huge issue, as many of those formulas can easily be bent to create over-powered items. Also, they can create items that are much too expensive for the effects they grant. Finally, magic items are noce, but what is really needed is flavorful magic items, which do more than give +2 to hit here and an extra fireball there. I feel I've managed to create those.
Also, Artifacts and Reilics are fun to crete, but certainly wouldn't be the focus of a book, as they are extremely difficult for DMs to simply toss into their campaign.
RE: New Creatures
We've got many new creatures created, including demons, devils, dragons, celestials, and at least one creature of every creature type. The hard part here is trying to ensure you don't end up with a book of great creatures whose CR causes entire parties to die or skyrocket levels because they were too high or too low.
RE: NPCs
We've also got quite a few NPCs in the works. Included are possible allies, possible foes, and even some that could go either way depending upon when the party meets them and under what circumstances. Every NPC developed would have at least three "snapshots": portraits of them at certain levels or CRs so they'll be easier for DMs to drop into their campaigns.
RE: Adventures
We've got tons of adventure ideas, but only one is anywhere near fleshed out, and really needs to be playtested. It is very story oriented, and doesn't involve a single underground complex.
All adventures I create for publication (if any) will most definitely have hooks to them, as well as being open-ended affairs wherever possible. It is incredibly difficult to put a truly free-flowing adventure into print, becuase without some flow of eents there is so much that can happen. However, it can be attempted, and even done well if enough care is given to it and thogut is used.
RE: Encounters
No solitary drop-in encounters have been made, but they certainly could be. What would people think about a book of encounters akin to the Book of Lairs from past editions?
RE: New Skills
I should have been more specific. We've only got one new skill in the works. Although I'd prefer not to give too much away regarding the rules of it, it is called Mana Weaving. It allows practitioners to create various magical effects without the use of spells. However, whenever the effect duplicates a spell, it is much less effecient than the spell itself would be. For example, you can weave force armor, but it takes at least five minutes, and doesn't last near as long as Mage armor, also, the duration nd effectiveness of the armor are determined by your skill check. Another minor use of the skill is to create and hurl bolts of mana. They only do 1d6 damage, and the skill check determines whether you hit or not. Additonally, spell resistance applies to many mana effects, and also counts as mana resistance of the same level for bolts (or other dmaging effects with the [Mana] descriptor, of which I have a few spells.
Retrofitting is incredibly simple, as it is a class skill for wizards, sorcerers, and Master Weavers (a Prestige Class devoted to the Mana Weave); cross-class for all other spellcasters; and excluded from the rest of the classes. Granted, it does become difficult for existing characters to change their skills to get it if they want it, but that is the case for any character who has chosen feats or spells before a new book comes out.
None of the effects are major enough to require a reshaping of the way the game world works.
Other Stuff:
Mass combat rules would be great, nd I plan on needing them for my campaign. However, I don't hve the time required to create such a large set of rules, so I'll be using a conversion of 2e's BattleSystem. I'd be willing to bet that publishing a conversion of that and selling it would get me a call from WotC's legal staff.
Ships and ship combat are about as far from "writing what you know" for me as they could be. I've never even been on anything larger than a speed boat.
Of course, the problem with "write what you know" is that I know gaming, and I can't very well introduce rules for PCs playing RPGs.
I've heard the mention of accuracy and playtesting from quite a few of you, and couldn't agree more. Anything that sees publication through me will be playtested AND easily meshed into the current rules set. That means that more than just my eyes will view the material before it goes out, and that it will have been used in at least one game, probably more.
As to why and how: The why is because I've got tons of ideas and would like to share them. I'd also like to see my name published and be able to say somebody bought my ideas. Heck, even just a few people thinking my ideas were worth purchasing would be nice.

It's most definitely a "striking out on my own" situation. I certainly wouldn't mind some d20 company out there seeing my work and asking me to freelance for them, but if that doesn't happen, I won't be crushed.
The how is: electronic format, for sale.
Well, I guess that's about enough out of me for now. Anyone else care to add something to the mix?
And one final note, don't worry, everything put out for sale will be better proofread than this post is.
Thanks,