I have been working on a basic idea for a world and a setting within that world, and I am looking for feedback. Keep in mind that this is designed as a D&D campaign:

Basic idea behind universe: Two conflicting cosmic forces, known as the Eldritch Gods and the Eldritch Demons (these are translations for mortals- they are not really "gods" or "demons", that is just the idea from the Eldritch Gods' perspective). They used to live in different galaxies, and when they collided, the Eldritch Demons attacked the Eldritch Gods. The Gods, being great cosmic builders, designed a vast number of planets to act as prisons. This accounts for any activity coming from the core of the world that this takes place in, as those are attempts to escape by the imprisoned Eldritch Demons.

Idea behind world (I don't have a name for it yet): When the planet was formed, there was a lot of raw ambient energy. This coalesced into many different beings, which led to the Age of Chaos, which lasted over 2 billion years. This age eventually ended when there were ~10 different races and ~30 gods, ~12 of which are major gods. Then came the Age of Gods, where the races were still experimenting with the different enrgies of the world, and gods were the major players in the events. Around 5 of them died in a rather large war, and the enrgy was distributed among the next generation of mortals. Then was the Age of Gods and Mortals, which had mortals directly interacting with gods and were more powerful. Many god-like people still are alive in the current age of the world, which is the Age of Mortals.

The Age of Mortals is a time where while there are still gods that are very powerfull, and are still major figures, but there are much mortals on the planet, so the energy is more spread out. This age has been around for ~2,000 years. The world is in the middle of a Renaissance of magic. Prior to this renaissance, magic users were often apprenticed under other mages, and were very confined to specific regions. Now there is more communication, and there are schools that teach magic being set up around many major cities. Mages become much more common. However, new ideas are still unfamiliar and innovations are not well known or implemented.

The magic system: Arcane magic is somewhat hard to channel. For this reason, there are not an enourmous amount of natural sorcerers around. However, during the Age of Gods and Mortals, an importan discovery was made: that they could say certain phrases that allowed spells to be cast at a much reduced cost. This allowed people without a large natural affinity for magic to be able to cast spells for fractions of the cost. Moreover, if they could decipher which phrases affected which parts of the spell, they could modify the spell to suit their needs or even create their own.
Divine magic: It is actually seperate from arcane magic. It is much more powerful, but cannot be harnessed through normal means. It is almost entirely distributed among the existing gods, but unlike arcane magic, it is more powerul the more spread out it is. The gods spread out their power to their followers, and thereby have a larger reserve to draw from. This power is also usable by the mortal in which it is stored.

Please give feedback on anything you feel it needs or could be improoved.

Rules specific:

My question here is: What's so different is magic system are you proposing that is from the 3.5e ruleset? You have given some principles, but does that mean a fighter that could grasp the phrases with skill points in spellcraft/knowledge(arcane) could cast crude arcane magic without taking up the arcane caster classes? In what forms would these crude magic manifest as? Will it trivialize the sorceror/wizard spell list?

Divine magic: How is it much more powerful? How do you define it in terms of rules? Are they say... 1.5 times better than enlisted in PHB?

Now what accessories do you allow in this campaign? The Complete books are so insane and useless at the same time they can easily break your whole idea on magic, and further break your campaign if not modified carefully.

Now more campaign specific:

Are you trying to surround your players with conflicts of battling ancient Eldritch demons? Are they players reside on the "planets" of their own or they are also living among these planet prisons? Their are various considerations for different cases. For former one, the planet prisons may have cultivated some civilizations surrounding the lore of Eldritch demons. Player interactions will be more like first contact to outsiders with very different cultures. For later one, the players are within these cultures, and they might adventure to serve these demons, either by their own choice or by malevolent manipulations. You will need a lot more in depth descriptions on these civilizations.

Well, last things first.
The people on the world are completely unaware of the fact that there are Eldritch Demons living underneath them, at least until the drow figure it out and become their servants in trying to break them out. I have yet to decide how other planets would work, but I think that they would also be unaware until directly confronted. Remember, people rationalize what they cannot understand.
Accessories:
Only core rulebooks, plus the UA Spell Points system.
Magic system: Yes, a fighter could learn magic if he wanted to. However, there is still an innate reserve of power necessary, so if they want to develop it, they need to multiclass. These might manifest (probably) as the Adept spell list.
Divine power is not more powerful rule-wise, but lore-wise it is. People with even a shred of their deities power can be insanely powerful, but that is why it is spread so much. A god, should they manifest all of their power, would probably be able to at least wound an Eldritch God/Demon.

By the way, I have come up with a bit more about power in general. Evil gods have more power, but are less likely to share. Good gods have less power, but share more. When a god dies, all of the power stored in mortals get released, and can then be tapped by other gods. Power, however, is very corrupting. An idea I had is that at some point, the characters will kill a god in a pantheon-wide war, but over time some of the previously good gods, drunk with their new power, fall and replace the old evil god.