|...Sol and Luna, hun. Fire Clan of the north still use Mars, after all. You've gotta keep the naming conventions in tact, otherwise it breaks away from the style of lore too much.
Inepts I could see being done, though. A Full Caster has three weapons, and a strong command of psynergy - A Physical Adept has four weapons, and a lesser command of psynergy (though equal in strength, due to mixing it with physical skills). Logically, that'd leave an Inept the ability to use Five weapons, and likely any armor type of their choice (save ones that are gender/race locked). Statistically, they'd be similar to warriors. However, they'd have higher stats in regards to HP to keep things balanced. You can still give them djinn, and it'll affect their class (they'd even have an elemental affinity to keep in mind, though they won't be able to utilize it), but they can't use djinn or summons, have no spells to cast, and can't trigger unleashes, only standard criticals.
So why choose them? Well, obviously because they'd have better criticals. Imagine this:
If the standard adept has a critical modifier of, say something like this:
A third of the adept's level is used as the "Critical Number (CN)". When a critical hit lands, it adds CN to the final damage, multiplies it by 1.CN, and lowers the enemy's defense by CN%. So if they were Level 36, the results would be:
Enemy's defense -12%
+12 Added damage
Final damage multiplied by 1.12
More damage for sure, but nothing really spectacular compared to an unleash. But in the hands of a pure physical specialist that is the Inept, it would be two thirds of their level instead of just a third - twice as strong on paper, though a bit stronger in practice:
Enemy's defense -24%
+24 Added Damage
Final damage multiplied by 1.24
At level 99, it would look like this:
Enemy defense -33%
+33 Added Damage
Final Damage multiplied by 1.33
Enemy defense -66%
+66 Added Damage
Final Damage multiplied by 1.66
Now, this isn't how crits work in either the GBA or DD versions of the battle system, but it is one I'm currently toying with. The improved critical hits allows inepts to strike pretty well compared to the might of unleashes, and if we were to set up an Inept the way I'm working them into the GSAR at the moment, then it'd allow them to not only be a very viable option in Golden Sun, but something to seriously take into consideration - because with crits like that, and with all their crit+ gear going into standard critical hits since they can't unleash, they're actually the go-to fighter for high-defense enemies thanks to that great defense piercing effect crits would have. This, of course, also assumes a system where enemy defense is actually respectable, which DD most certainly did not have.
If we were to look at actual values, let's try this: Use the 999 cap of the GBA games, and the Great Dragon's 300 defense, and have the attacker be Level 99 (enemy level is irrelevant in these calculations). We will compare a Inept Critical to several unleashes:
Normal Damage: 349 damage, no elemental modifier (unless weapon elemental)
Adept Critical: 574 damage, no elemental modifier (unless weapon elemental)
Inept Critical: 854 damage, no elemental modifier (unless weapon elemental)
Megiddo: 1048 damage, +/- up to 50% due to Venus influence
Legend (Single): 422 Damage, +/- up to 50% due to Jupiter influence
Legend (Triple): 1267 Damage, +/- up to 50% due to Jupiter influence
Vengeance (Single): 422 Damage, +/- up to 50% due to Venus influence
Vengeance (Double): 845 Damage, +/- up to 50% due to Venus influence
Vengeance (Triple): 1267 Damage, +/- up to 50% due to Venus influence
So those "big three" (or big four, if you add centurion, which could be argued to be even stronger) are still very powerful, but look at the Vengance values - it's on par with Vengance's double trigger damage - in fact, likely exceeding it, given that Venus is the rarest weakness in the game. Plus, also consider that these unleashes are still somewhat OP, and that in a more balanced game the multipliers they trigger would be scaled back (to say, 250% for Megiddo, 1x or 2x for Legend, and 1x, 1.5x, or 2x for Vengeance) and that means an Inept crit would be on par with an uber unleash at max level, effectively making them an even match statistically for an adept, and balancing them on paper.
Whether or not this follows lore, however, is another story entirely - it all depends on how they build it from here. Let's hope they don't just make inepts some useless normals.
Light and Dark, though, I see as being not a primary affinity like with the four elements, but rather a secondary affinity. This means you'd be associated with both an element and a fundament. This has its pros and cons, and has to really bend the lore that was introduced in Dark Dawn a bit in order to work well, but unlike DD's version they don't outright break the lore of the first two games. You can be both a Luna adept AND a Mercury adept - an in this case, what you'd be doing is specializing in Maledictions and the Lunar Aspect of Mercury (unless you use an alt class, of course - then you'd use the Lunar Aspect of those elements as well).