Iron Heroes was a d20 game and like Arcana Unearthed, using the rules in d&d was not really suggested or encouraged. I tried using AU' s casting rules in a 3e game and it made casters broken but the system worked much better when it was self contained as an AU game
Iron Heroes was its own game. However, I specified the Book of Iron Might which is a supplement for D&D from the same publisher. It focused on martial combat and allowing martial characters do cool stuff. I found the maneuver system in that supplement much better than the one in Tome of Battle. Plus, it did not add new classes.
And again, I did not say Arcana Unearthed which is also from the same publisher as Iron Heroes and the Book of Iron Might. I specified Unearthed Arcana which was an official 3e D&D supplement of optional rules
The core doesn't really support modification beyond that core aside from suggestions, scant few, especially in 3e and 4e. Yes variant rules supplements exist but the core brand, aside from the UA supplement supplement and the HR, but the core d&d brand didn't go outside those assumptions and using it as a toolkit involved heavy kit bashing of the game as written with little advice supported.
Let's see the suggestion and variants
3e PHB
Tailoring the character (creating class variants): Discusses tailoring the Fighter into an ex-bodyguard for a Thieve's Guild (a revised version appears in 3e Unearthed Arcana)
3e DMG Variants: 33 rules variants plus advice on tailoring races, advice on creating new classes and tailoring new ones, firearms and asian weapons for those gms that want to introduce them
I. Chapter 2: Characters- Race
The section on races discusses altering existing races and creating environmental subraces (the latter is fleshed out in Unearthed Arcana)
1. Variant: Monster as Race
II. Chapter 2: Characters- Classes
The section discusses creating classes and tailoring existing classes. In addition to the variant rules listed below, it discusses turning the ranger into an undead stalker with sneak attack vs. undead and paladin smite evil vs. undead only. (A few class variant examples appear in early 3e supplements, but it does not really get truly supported with plenty of examples until Unearthed Arcana and later supplements)
2. Variant: Spell List Variant Spellcasters (covers tailoring divine spell lists to deity, arcane lists to schools, and lists for other variant spellcasting classes). They even provide a Witch variant as an example.
3. Variant: Multi-classing at 1st level (covers apprentice level characters and 0/0 level multiclassing at first level).
III.Chapter 2- Characters- Leveling
4 Variant: Learning Skills and Feats (learning skills and feats requires Trainers, training time, and training cost. Also, increasing existing skills requires a proper environment and opportunity to have practiced and applied the skill)
5. Variant: Learning new spells: wizards finding spells, sorcerers learning from patrons, bards studying with other bards
6. Variant: researching spells (creating new spells)
7. variant: gaining class abilities (requires a trainer of the same class and training time or the character has to spend twice as much time on his or her own self training)
8. variant: general down time
9. variant: fixed hit dice
IV. Chapter 3: Running the Game- Combat Variants
10. Variant: Surprise Round
11. Variant: Roll initiative each round
12. Variant: Automatic Hits and Misses
13. Variant: Defense Roll
14. Variant: Instant Kill
15. Variant: Softer Criticals
16. Variant: Critical Misses (Fumbles)
17. Variant: Firing into a crowd
18. Variant: Clobbered (taking half of current hit points from a single results in character taking a partial action on next turn)
19. Variant: Death from Massive Damage Based on size
20. Variant: Damage to Specific Areas
V. Chapter 3: Running the Game - Monster Ability Variants
21. Variant: Separate Ability Loss (each 2 points of ability damage, the character takes a -1 penalty to ability related rolls
Non Magical Psionics Variant.
22. Variant: Non-magical Psionics
23. Variant: Characters with Scent
VI. Chapter 3: Running the Game - Skill and Ability Check
24. Variant: Skills with Different Abilities
25. Variant: Critical Success or Failure
VIII. Chapter 3: Running the Game- Saving Throws
26. Variant: Saves with Different Ability Scores
IX. Chapter 3: Running the Game- Adjudicating Magic
27 Variant: Spell Roll (Save DC = d20 roll + spell level+ caster modifier)
28. Variant: Power Components (alternative to spells casting XP)
29. Variant: Summoning Individual Monsters (affects Summon Monster and Summon Nature's Ally spells)
X. Chapter 5: Campaigns
30. Variant: Upkeep (characters are charged a monthly fee based upon their lifestyle)
Not a variant, but there is sidebar suggesting cool ways to break the rules and tailor the chararcters to your own setting
XI. Chapter 6: World Building
Not a variant, but suggestions on building a different world talks about Asian Weapons, Firearms, and Futuristic Weapons. With sample weapons.
Also, the section on Differing Magic, briefly discusses changing the level of magic to either low magic or high magic
XII. Chapter 7: Rewards
31. Variant: Faster of Slower Experience (controlling the speed of leveling)
32. Variant: Free Form Experience (instead of calculating XP, hand out recommended XP based on encounter toughness
33. Variant: Story Awards: Non Combat encounters, Mission Goals, Rolepalying Wards, Story Awards