Outside of splatbooks, Dragon Magazine is your best bet for filling in the gaps, unless you want to create your own homebrew versions of e.g. the Sentinel's missing seasons. I once did a few homebrew weapon specializations for the Slayer, and it wasn't that difficult.
Essentials has left so many gaps I doubt Dragon will fill them all. The acolytes of the missing seasons have had rumours of coming out someday for a while, I think holding your breath for any specific gap is ill advised. I don't think they ever intended to fill all the obvious gaps....there are just too many.
Gaps I see:
-Knight, slayer, scout: different weapon types than just sword and axe (sword and hammer for the knight)
-mage: plenty of schools left to dig into: conjuration, transmutation, plus all the elemental possibilities beyond pyromancer.
-warpriest: you could as a full time job write domains for the next year and still not finish most peoples list. Editorial: Making each domain have 11 unique powers on top of essentially unique PP and abilities at various levels is neat for the first few times you do it, and then a horrible idea for the rest as the number of cleric spells goes nuts for anyone making an O-cleric. If you have the compendium look up warpriest and just scroll down through the bloat. And yet I WANT ALL TEH DOMAINZ MAYDE NOW!!!
-Druid Sentinal: two more seasons beg for life, and about a dozen possible animal companions (I think the fey beast tamer's flock should be addable to the list with appropriate improvements)
-Hunter: Thrown weapons, slings.....i mean, some one probably thinks these are gaps worth filling, right?....anyone? No? *shrug*
-cavalier/blackguard- plenty of vices and virtues out there. Greed, charity, Love, Hate, sloth, diligence, despair, hope, peace, war, not to mention ones of specific deities like Neverwinter introduced for warpriest
Honsetly [to me] the only Essential classes that don't have obvious gaps are the Thief (what more could they possibly need....less power maybe?), and maybe the hexblade (it has five builds [fey('winter'), fey(white well), infernal, star, and gloom] already, though could EASILY be expanded to add vestige and sorceror-king pacts). At some point many of these stop being gaps, and are instead actually just mechanics that inspire many many possibilities (domains, virtues, vices, pacts for example, I get very carried away dreaming of all the possibilites here), but some are obvious holes (Hammer slayers and Axe Knights = duh!).
I think that having 'big gaping' gaps is "acceptable", but not ideal, and I think 'they' feel the same way, but have limited power to devote to this gap filling, or they tried and the results were crap. They've stated many times they want to focus on A) good flavor B) what people _actually_ want. Just because I want a poison domain for death priest doesn't mean more than three people will ever play what costs them money to publish and dilutes the product to some degree. Just because you can make a warpick based slayer doesn't mean it will spark anyones immagination enough to make them make sense enough for someone to base a character around. Essential classes have the goal of getting ~2 flavors of each class out there, make them playable and inspiring. If the class never gets touched again, they still have enough to keep people playing them (and if people aren't playing them, adding gap filler won't make them a better class....look at the reaction to the fey pact binder).
Homebrewing is the simple answer (Its what I do when I feel the cold gaps in essentials), I don't know if any third parties are able to contribute. Two of my submissions (rejected on ground of not compelling flavorwise) were essentially gap fillers for the scout and cavalier/blackguard. It is something I want out of 4e, and I'm willing to do the work for it.....I just wish I could convince people to let me bring it to the table at D&D Encounters or campaigns i don't run....
Bottomline (for me): 'I WANT GAPS FILLED! SOOOO MUCH COOL STUFF!' _and_ 'It ain't reasonable (for WoTC) to fill all the gaps to even the least amount desired by me' We aren't being cheated out of Essential gaps being filled by the new eddition announcement, we're being cheated out of it by the cold harsh realities of life.
