Remember, everything in game (skills, classes, races, whatever) is changeable by the DM. Everything coming from WotC or Dragon Magazine is just inspiration of the story you exist in or are telling. If you think tempest is mostly within the flavor of your character, change the bit that isn't. Since you aren't taking the Spring Attack Tree(SAT), but you are taking the Improved Two Weapon Fighting Tree(ITWFT), build or rebuild based around that.
These are a few concepts I have rolling in my head for a Tempest minus SAT. Since they are missing SAT, it makes no sense for them to the fifth level bonus feat of Two Weapon Spring Attack.
Cleaving tempest:
Requirement: Power Attack, Cleave, (maybe great cleave), ITWFT, BAB=6+.
Supreme Cleave (Ex) said:
Beginning at 5th level, an Cleaving Tempest can take a 5-foot step between attacks when using the Cleave or Great Cleave feat.She is still limited to one such adjustment per round, so she cannot use this ability during a round in which she has already taken a 5-foot step.(Copied from Complete Warrior page 35)
Ranging Tempest:
Requirement:2 Favored Enemies, ITWFT, BAB=6+.
Additional Favored Enemy said:
Level 5:Just look at the ranger entry in PHB1. Treat it exactly the same way. (PHB1, page 47)
Flanking Tempest:
Requirements:Weapon finesse(Some other stuff?), ITWFT, BAB=6+.
:Improved Flanking (Ex) said:
A Flanking Tempest of 5th level who is flanking an opponent gains a +4 bonus on attacks instead of a +2 bonus on attacks. (Other characters flanking with the Tempest don’t gain this increased bonus.(copied from Complete Warrior page 12)
Note: These have not been tested for balance, clear everything with your DM before planning on it. Make any additional changes you wish.