One of my favorite strategy games was called Alpha Centauri, by Sid Meier. It was so interesting because of the varied methods one could pursue to achieve victory.
1. Total Conquest. You (and those who are allied in a pact with you) must be the last ones standing. In essence, if you don't agree with me, my friends and I will kill you.
2. Diplomatic victory. You would have to be so well-loved (and have so many people) that in a meeting of the Planetary Council, you would propose that the other factions vote you supreme leader. This method could be done in two ways: paying others off to vote for you, or by being so powerful (wiping out those who would vote 'no' or having so many people to constitute a massive vote) that they could not deny your wisdom.
3. Economic victory. You corner the 'global energy market', in essence, you hold such a monopoly on resources and wealth that none can stand against you. The economy has already been debated in other topics, but if someone totally and completely rules the economy, even if they are faced by powerful magic, they would probably be able to do some horrid things (if your people don't get any trade or money because another faction is angry with you, they may try to revolt). This way, a spellcaster could pursue spells that would ultimately result in an enhanced economy.
4. Scientific victory. Trade off 'scientific' for some other method. For example, give each 'sphere' of magic its own spell of ultimate destruction or perhaps there would be some inter-sphere spell of ultimate destruction (you must pursue knowledge and influence through each elemental power to gain mastery over the final spell) and by achieving this spell, you could either call the game an auto-win, or continue playing and actually wreak havoc with the ultimate spell. This would be a method more inclined to the studious, who would have to do a lot of research to find all of the nit-picky details about the spell.
5. Artifact victory. Something like the Tear of Asha from Heroes of Might and Magic V. I don't like the idea of everyone knowing right away what it is (based on the context, if everyone just randomly discovered a legendary artifact and knew exactly where it was, it wouldn't make a vast amount of sense) and people could have various methods of discovering the location. Do you go to a sooth-sayer or diviner, perhaps the black market, and pay for information? On that hand, will the sooth-sayer/diviner tell you the truth, or will their visions be incorrect? And if a bounty goes out for information, will they really find it, or try to ruin your efforts? Perhaps explorer-types could seek out altars or signs or some other sort of revealing-devices to locate the artifact. Those who are powerful in the ways of magic and research could delve deep into their books, trying to locate the information based on what they have in their libraries. Even a sort of spell that reveals locations (IE: cast a divination spell, and it reveals a shroud of the Fog of War for a turn or so). Those who want to pursue a mixture of means could do so: they would ultimately lose out in those departments (devoting wealth from one area to another means that one area loses out; research and spells take away from their areas, too, and exploration will take time and efforts.)
Other small details...
Instead of razing a node, perhaps it ought have its own unique method of destruction. I mean, if I were a magician, I wouldn't want to smack the source of my vast power with a hammer. Perhaps the magician would have to come -- personally -- to the node, and then drain its essence. Draining a node would give your magician enhanced power (more points to cast with, quickened casting time for spells corresponding to that node, etc.) but it would be very temporary. Say, after ten turns, it will vanish. This could give nodes an alternative sort of use: if you are under a great deal of stress, you could drain the node and gain vast power, at the cost of permanently losing that node. Dropping the bomb, so to speak.
Alliances...again with Alpha Centauri. Alliances were very tricky things in AC - if you sided with person A, or were even friendly with them, then their enemies (henceforth 
would start to hate you. If A and B went to war, A would come to you and absolutely demand that you help, otherwise they will begin to hate you. But, on the other hand, alliances would open up a great deal of trade, give you line of sight into allied territory, and allow you to collaborate battle plans (Alright A, you plan to attack B's trade outpost? I think you should focus on his capital; leave the outpost to me.) which would add depth to the game (like battle-plans in Empire Earth) by allowing you to plot and plan conquering enemy territories. Plus, the longer you go as allies, the more they'll like you.
At the same time, breaking an alliance would probably be a big no-no. Basically, you say "A, you're a bad person, and I don't want to be associated with you." While it probably would not (on its own) lead to open conflict, it would certainly make people not like you a great deal. Plus, breaking alliances often would incur a diplomatic penalty: keep doing it, and people would get wise to not trust you.
