So... something like this?
The MetamageHD: d8
Saves: Poor Fort, Poor Ref, Good Will
Lv Base Attack Special 0th 1st 2nd 3rd 4th 5th 6th 7th 8th 9th
1. +0 Bonus Feat 3 1 - - - - - - - -
2. +1 Bonus Feat 4 2 - - - - - - - -
3. +2 5 3 1 - - - - - - -
4. +3 Bonus Feat 5 4 2 - - - - - - -
5. +3 5 5 3 1 - - - - - -
6. +4 Bonus Feat 6 6 4 2 - - - - - -
7. +5 6 6 5 3 1 - - - - -
8. +6/+1 Bonus Feat 6 6 6 4 2 - - - - -
9. +6/+1 6 6 6 5 3 1 - - - -
10. +7/+2 Bonus Feat 6 6 6 6 4 2 - - - -
11. +8/+3 6 6 6 6 5 3 1 - - -
12. +9/+4 Bonus Feat 6 6 6 6 6 4 2 - - -
13. +9/+4 6 6 6 6 6 5 3 1 - -
14. +10/+5 Bonus Feat 6 6 6 6 6 6 4 2 - -
15. +11/+6/+1 6 6 6 6 6 6 5 3 1 -
16. +12/+7/+2 Bonus Feat 6 6 6 6 6 6 6 4 2 -
17. +12/+7/+2 6 6 6 6 6 6 6 5 3 1
18. +13/+8/+3 Bonus Feat 6 6 6 6 6 6 6 6 4 2
19. +14/+9/+4 6 6 6 6 6 6 6 6 5 3
20. +15/+10/+5 Bonus Feat 6 6 6 6 6 6 6 6 6 4Class Skills (4+Int): Concentration, Craft, Decipher Script, Disable Device, Knowledge (Any), Profession, Search, Spellcraft, and Use Magic Device.
Proficiencies: All simple weapons, all martial weapons, light and medium armor*, no shields.
*A Metamage can cast spells in light and medium armor ignoring arcane spell failure.
Spellcasting: Spontaneous from his spells known. A Metamage begins play knowing only a single 0-level spell. At 3rd level he learns a single 1st level or lower spell, at 7th level he learns a 3rd level or lower spell, at 11th level he learns a 5th level or lower spell, at 15th level he learns a 7th level or lower spell. A Metamage never learns any additional spells. Even though he is a spontaneous caster, the Metamage is able to apply metamagic feats to spells he casts without increasing the casting time. A Metamage gains bonus spells per day, and the save DCs of his spells are determined by his Intelligence modifier.
Bonus Feats: At 1st level and every even level thereafter a Metamage gains a bonus feat from the following list: ANY METAMAGIC FEAT, Easy Metamagic, Innate Spell, Obtain Familiar, Practical Metamagic, Metamagic School Focus, Metamagic Spell Trigger, Arcane Thesis, Arcane Toughness, and Arcane Consumption.
-------------
I decided to put UMD in the class skills because I figured it could use the help at lower levels, and at higher levels it really isn't going to matter. The class seems to lack any sort of flavor, at least to me.