Warmains are tank specialized fighters with less flexibility in choices. They out tank a fighter going for the same build but if you want an archer or to multiclass some warrior concept into another build you want fighter.
I've played a champion, giant racial classes, and a warmain as a PC in D&D games and it went fine. I've used unfettered as an NPC, I like them best of all the various
3e swashbuckler type options. Giants you have to consider whether you want size large at level 3.
I wouldn't worry about 3.5 vs AU magic systems. They are class specific systems so it is like allowing psionics, warlocks, binders, etc in the same game.
Akashics are basically skill monkey rogues without all the sneak attack and with more bardic knowledge, no imbalance except I feel they are weak.
Going with 3.5 skill set is easy to convert.
You have to decide whether you are giving the bonus level 1 feat, I'd guess not as you are sticking more on 3.5.
Having multiple overlapping classes for similar archetypes is fine, monk and oathsworn = 2 separate martial arts traditions; unfettered and swashbucklers = different sword schools; champions and paladins could be god specific, warlocks, wizards, magisters, psions = different magical schools; clerics, greenbond, druids, favored souls = four different paths to divine magic, etc.
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