Elegance is doing much with minimal energy.
I believe the strongest game system would be one that functions perfectly with basic rules, but also has the option for adding expansion rules for those who want more complexity.
The core mechanics would be easy. The baseline would be classless, but classes would be a big part of the game, because classes help you fulfill certain classic archetypes of fantasy. The one base class would get 10 hit points, 8 skill points, and 8 character points at 1st level. Each level thereafter they'd get 4 hit points per level and 2 skill points, plus 6 character points.
- A feat costs 2 character points.
- Increasing base attack bonus costs 4 character points.
- Hit points cost 1 character point per 2 HP.
- Skill points cost 1 character point per 2 skill points.
- Increasing your caster level costs 4 character points.
Spellcasting.
Each day you gain Magic Points (MP) based on your caster level. Spells cost a certain number of MP, and you cannot spend more MP per spell than your hit dice. The simple version of the rules just has basic spells that you learn like a sorcerer. The more advanced rules let you create spells on the fly, and give you options for differentiating different types of magic.
Because MP is based on caster level, but the maximum MP you can spend on a given spell is based on Hit Dice, you can multiclass to get a few levels of spellcaster and still be able to cast some worthwhile spells, just not many per day.
Combat.
This isn't my field of specialty, but I'm sure it could be simplified a bit. One thing I recommend considering is, in addition to hit points, we also have will points. Mental spells sap will points. This makes controlling someone's mind a little less "save or die." I'm not quite sure how to work it, but I don't like that a typical 20th level wizard can resist
dominate person most of the time, but by random chance his will might break when confronted by a simple
charm person.