Alternate magic systems and options, also E6

[Komatik]

So, long story short, I love warrior-mage types, I love magic. I adore options. Many of my friends love a bit lower-magic games of grimness and grit.

Given the above, I love many alternate magic systems to bits - invocations, Incarnum, sweetbabyjesus PACT MAGIC. Nice stuff, great flavour, better balance, all that jazz. The one issue I universally have with them is that they give out power slots very sparsely - simultaneous vestiges, binds, invocations known numbers are all low, low, low. Given the seemingly overall lesser power level these systems have to begin with, it just seems like cruel and unusual punishment when versatility is the main draw of magic in the first place.

E6, while otherwise amazing, doesn't help. A single vestige a day? 3-4 invocations known? It just seems poor.

Incarnum faces a different issue, namely that being able to never reach most binds just seems wrong in a way spell level or vestige level advancement stopping at level 3 never could, because they deal with physical locations and things.

So, given the above:
Would Warlocks/DFAs/Binders become broken if they had more invocations known or could bind more vestiges a day? Still the low-level stuff, obviously, just more of it for added versatility.

For Totemists, how much of their power is in the number of simultaneous binds, in essentia, in open chakras. That is, would a totemist with unusually many open chakras but the typical limited number of simultaneous binds and small essentia pool be more or less okay? Or do things like a throat bind for example confer something so broken it probably shouldn't be allowed?

[Lonely Tylenol]

Feats help with both of these quite a bit. Extra Invocation can be used to expand your repertoire of least invocations, and the Bind Vestige line can allow you to bind additional vestiges for individual powers (not for the full breadth of the vestige on a per-feat basis, unfortunately). But no, broadening access won't kill you either.

You're getting into homebrew territory with this, beyond E6 modifications even, but here are a few suggestions:

Epic feats
Warlock - The Warlock can take one lesser invocation known for every two least invocations gained through the Extra Invocation feat. (So it would go Least - Least - Lesser - Least - Least - Lesser.) You set the maximum.
Binder - The Soul Binding (two vestiges) ability of an 8th-level binder can be its own feat. The ability to bind fourth-level vestiges is also an ability worthy of a feat, but it might be a good idea to include caveats (the binding check is much more difficult, or the consequences for failure steeper, or something of that sort). This can be extended to the Dragonfire Adept as well.
Shadowcaster - A feat to treat fundamentals as at-will abilities, a feat to treat your Shadowcaster spells as spell-like abilities (complete with one more daily use each), and a feat to learn an initiate-level mystery (which you can take multiple times, up to the number of completed paths known) all sound like worthy ventures for the single-classed Shadowcaster in your E6 group, because let's face it: they've suffered enough.
Incarnum classes - give the Open Lesser Chakra feat a requirement of "Totemist, Incarnate or Soulborn 6". Now, these chakras are open only to the dedicated incarnum classes.

In addition, you could think of giving the Warlock blast shape and eldritch essence progression like a Dragonfire Adept, or something similar. I usually recommend one at 1st level, plus one for every 5 class levels (much like the Wizard feat progression). This seems like a relatively reasonable fix: now, if you're not combat-inclined but felt obligated to pick up a few Eldritch Blast-themed invocations, you can still have a few at your disposal, while using all your normal invocation slots for things you actually wanted.

[Bonzai]

I am running a homebrew campaign and two of my players are playing homebrew classes as well. One of which is a re-flavored Warlock that is essentially all about wind magic. Since I did a lot of swapping around with the invocations, I allowed him to know them all when he reaches the appropriate tier instead of learning them one at a time. The results were that the character is very strong, but no more so than a beguiler or dread necro for the most part.