Some general ideas I've come up with

STATS: There's six of them, split into two categories: physical & mental.

PHYSICAL -

STRENGTH --
a measure of physical force which can be exerted. For example, power of attacks, ability to lift heavy things, amount of things that can be carried, ability to overpower people, etc.
DEXTERITY -
a measure of nimbleness, speed, and preforming tasks which require percise movements. For example, picking locks, picking pockets, dodging attacks, moving quickly, climbing things, etc.
CONSTITUTION -
a measure of physical endurance and the amount of physical force the body can take, a measure of toughness and fortitude. For example, running long distances, drinking a lot of ale, holding your breath, breathing easily on a high mountain, resisting being knocked back, etc.

MENTAL -

INTELLIGENCE --
a measure of the ability to store, recall, and apply knowledge, as well as a measure of the force a mind can exert. For example, casting spells, remembering things, solving problems, perception of details, finding patterns, and making magic more powerful through the mastery of magical techniques.
CHARISMA --
a measure of the force a personality can exert. The ability to charm, decieve, intimidate, and talk your way out of things, to be able to extract information through intimidation or persuasion, the ability to barter prices, the ability to inspire or destroy confidence.
WILLPOWER --
a measure of how much force the mind can take, as well as the ability to remain focused in trying situations. A measure of mental fortitude. For example, making saving throws against spells, the ability to precieve things that happen quickly during combat, the ability to remain composed during trying circumastances, etc.

STATS AT A GLANCE -

STR - Landing strong hits and weilding heavy things.
DEX - Landing many hits and moving quickly.
CON - Able to take many hits and outlast opponents.
INT - Arcane magics, finding enemy weak points.
CHR - Manipulation of minds through diplomacy, intimidation, deception, etc. Mind altering magic.
WIL - Manipulation of the physical world with the mind. Focus.

Combat bonuses -

STR - Damage bonus, ability to wear heavy armor, wield heavy weapons
DEX - Attacks per turn bonus, def bonus through dodging
CON - HP bonus, knockdown resistance
INT - Crit chance bonus, stealth bonus
CHR - Enemy debuffs through taunting, ally buffs through rallying
WIL - "To hit" bonuses, spell resistance


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CLASSES: Each class has two main stats.

BASE CLASSES -

WARRIOR - STR & CON (physical class)
ROGUE - DEX & CHR (mixed class) (magic-user at high levels)
WIZARD - INT & WIL (mental class) (magic-user)

ADVANCED CLASSES -

PHYSICAL -
BERZERKER - STR & DEX
MONK - DEX & CON

MIXED -
BATTLEMAGE - STR & INT (magic-user)
KNIGHT - STR & CHR (magic-user at high levels)
SPELLSWORD - STR & WIL (magic-user)
THIEF - DEX & INT (magic-user at high levels)
ASSASSIN - DEX & WIL (magic-user at mid levels)
WITCH - CON & INT (magic-user)
PILGRIM - CON & CHR (magic-user at high levels)
WARLOCK - CON & WIL (magic-user)


MENTAL -
ILLUSIONIST - INT & CHR (magic-user)
PSION - CHR & WIL (magic-user)

Advanced classes may only be chosen by multi-classing into them at fifth level. Alternatively, an advanced class may be chosen at first level if stat requirements are met. Multi-classing may only be done into a class sharing a main stat that is already possessed by a class a character already is.


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CHARACTER CREATION:
Each player begins with 500 XP to spend on their character's skills, plus a 1d6x100 roll. Each
point of INT above ten grants another 100 to spend. Vice versa for points below ten. Each base
class is free to become, but each advanced class costs 1000 XP. Extra XP points left over can be used to purchase skills.

LEVELING:

Class Skill Trees -

Each class has it's own set of skills that can be learned through the spending of XP, a sort of skill tree. Once purchased, a skill takes 1d6+5 days to learn. Each point of WIL above ten decreases that number by 1, vice versa for points below 10.
All skills of a certain level in a skill tree must be purchased before a character can be considered a level higher in a class and be able to move up to higher level skills.

(There will be other trees outside of the class trees that have to do with skills such as certain
fighting styles, "ranger" type skills having to do with animals, crafting, etc.)

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MAGIC: Certain schools can only be cast by magic-users with corresponding main stats. For example,
a spellsword can only cast Entropy spells, a wizard can cast Arcane, Entropy, and Creation spells,
a thief can cast Manipulation spells but only at high levels, and a warrior can never cast any
spells.

SCHOOLS OF MAGIC -
INT - Arcane
INT & CHR - Illusion
CHR - Manipulation
CHR & WIL - Psionics
WIL - Entropy
INT & WIL - Creation

SPELLS -
ARCANE --
- Anti-Magic Sphere (5e)
- Arcane Eye (5e)
- Astral Projection (5e)
- Bless (5e)
- Comprehend Languages (5e)
- COunterspell (5e)
- Dispel Magic (5e
- Find the Path (5e)
- Etherealness (5e)
- Dimension Door (5e)
- Death Ward (5e)

ILLUSION --
- Blur (5e)
- Dancing Lights (5e)
- Darkness (5e)
- Disguise Self (5e)
- Faerie Fire (5e)

MANIPULATION --
- Beacon of Hope (5e)
- Charm Person (5e)
- Command (5e)
- "Dominate" Spells

PSIONICS --
- Dream (5e)
- Foresight (5e)

ENTROPY --
- Any spell that destroys things, "fireball," "disintegrate," "cone of cold," etc.

CREATION --
- Cure Wounds (5e)
- Any spell that can create physical objects out of thin air, summoning spells, etc.

More spells to come soon, I've only made it through "G" in the 5e manual and hope to pull
spells from other editions of D&D and other RPG sources.

IDEAS -

Charge spells: Spells that increase in power as their casting time increases. Able to cast immediately for weak but quick spell, or able to charge for a powerful but lengthy cast time.

Sustained spells: Spells that provide a buff or debuff as long as they are being cast. Requires
constant concentration.

Counterspell: If someone casts a counterspell while a magic-user is casting, the casting is interrupted. If the counterspell succeeds at the same time the spell is actually cast, the spell affects the spellcaster instead, or if it is a positive spell, the counterer. In this way, spells can literally be countered or stolen, but there is only a small chance of this happening.


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COMBAT:

Combat is comprised of turns, each lasting three seconds. At the beginning of combat, before combat actually begins, each character in combat makes an initiative roll on a four-sided die. The number on the die represents the amount of turns it takes for a character to get ready and be able to engage in combat. The minimum roll is one because that is the amount of time it would take to draw and ready a weapon. Higher rolls (like a roll of four, being twelve seconds), would represent being unable to bring oneself to attack or being unable to choose a target.
One's DEX modifier is a direct counter to the number rolled, but the minumum on an initiative roll will always be 1. This can be avoided by already having one's weapon drawn before combat, meaning that it is possible to avoid initiative penalties altogether.
The state of being surprised incurs a +3 penalty to an initiative roll. An initiative roll may go above 4 if this happens. One's WIL modifier is a direct counter to this penalty, so that it may be negated entirely, but may not go negative.

Once initiative and surprise is determined, combatants must declare what they will do every turn.



IDEAS -

Combat lock: (Melee only) When two characters attack eachother at the same time, both must roll to retain focus (WIL check, WIL is short for a stat called Willpower). If both fail or if one fails, both just hit eachother. If both succeed, combat lock is initiated, and both may attempt grapple, dodge, feint, overcome, etc. Both characters continue to roll WIL checks every turn to retain focus and preform actions until one of them fails the WIL check or until one of the characters succeeds in one of their actions, breaking the combat lock.

Combat lock funtions as such: The two combatants both realize they are about to attack eachother, and may choose to either continue their attack, or stop their own attack to dodge the enemy, grapple the enemy, disarm the enemy, overcome (tackle) the enemy, etc.

It could play out like this: Two players declare to attack eachother at the same time. As both go in for an attack, both realize the other is also attacking, and one of them decides to dodge the other. The other notices this dodge, and decides to feint an attack on the dodger's other side in hopes to make him dodge the other way. The dodger recognizes this feint, and goes to grab the feinter's sword arm in an attempt to disarm. The feinter fails a reflex save, and his arm is grabbed. He attempts to punch the grabber with his fist. The grabber fails a reflex roll, and is punched, losing his grip on the arm. He also fails a fortitude save, and is knocked to the ground. On the ground, he fails his WIL save and his concentration is broken, ending the combat lock and giving the next turn to his opponent.


Psionic lock: If two psions attempt to attack eachother at the same time, both are put into psionic lock. During this special circumstance, both must make intelligence checks to attack and will saves to defend. If one of the psions fails a will save, both the spell they were casting and the spell being cast against them affect them, plus damage equal to the winner's INT stat.