Well, make that a 10 HD Fireball for 15 dice Empowered
Fiery Burst (Reserve Feat from Complete Arcane): Increase Caster Level by 1 for Fire Spells.
Arcane Thesis (PHBII): Chose an Arcane spell you can cast. When casting this spell, you gain the following benefits: 1. effective Caster Level is +2; 2. when you apply a Metamagic to your Thesis spell, it increases the spell level by one less.
You may take this feat multiple times, each time with a different spell.
Metamagic School Focus (Complete Mage): Benefit: Choose a school of magic for which you have the Spell Focus feat, or the school in which you have specialized. Three times per day, you can reduce by one level the cost of a metamagic feat applied to a spell of the chosen school. If you prepare spells, you can have only up to three such reducedcost
spells prepared at any time.
Special: A wizard can select this feat as a wizard bonus
feat. This feat can be taken more than once. Each time you take it, it applies to a different school of magic.
Voila, 9th caster level

If we want the 10 levels, we add in a feat from Players Guide to Faerun:
Spell Thematics: Your spells have a visual theme, such as ‘sphere’ or ‘lightning’. Spells you cast have special effects based on your theme, such as Summon Monster I manifesting as the creature springing from a thrown sphere. 1. Since your spells look different, Spellcraft checks to identify what spell you are casting are at +4 DC; 2. For each of your current and future caster levels, you may designate one spell as a ‘thematic spell’, which is thereafter cast at +1 effective Caster level.
Note: you cannot make your spell invisible & the effects of your spells are not changed in any way.
Point is, a lot of what the feats that have been suggested for E6 to provide certain benefits for E6 characters are already out there, by Wizards themselves (which only makes E6 all the more genius IMO

)
I'm positive we could do the same for Rogues, Clerics, Fighters and so on. And that's really what my suggestion was all about. Creae an appendix listing all feats that can't be included, but which are perfect for E6, perhaps even giving advice on 'Paths' using the feats, or at least what level 6 boundaries can be ..well, overcome, for lack of a better term.
Edit: Forgot to add: None of those have prereqs a 6th level character can't have.
Edit 2: I do think that melee'ers will need access to some damage dealing (the BAB +8 feats) still.