Magic. What would a high fantasy world be without it? Not much other than standard realistic fiction. On the other hand we have science-fiction, a world where technological wonders are prime. Then there is science fantasy. It's very difficult to combine the two genres successfully as each retains inherent values that may conflict with that of the other. The most important aspect of blending the two is describing some sort of magic or other supernatural force in such a way that it fits in with the story well. That's the problem we have: how do we integrate magic?
One way this conundrum could be resolved is by simply saying magic can do whatever the user desires. While the simplest solution it presents problems. If magic can simply be bent to the user's will, what is to stop a particularly strong person from utilizing the character in such a way that others could not counter. For this reason we must have limitations of some sort. But what kind of limitations? Should magic be restricted to such an extent that it can only be employed for paltry uses and illusions, accessed by a few weak users that hold almost no control over the source? Such heavy-handedness would make magic almost non-existant in the RP and therefore remove the 'fantasy' status.
There must be some sort of middle ground and to that I think it should lean more towards a free-form magic in which the user can determine what happens to an extent. Magic in and of itself should be an infinite supply of some sort, similar to the Force in Star Wars or True Source in the Wheel of Time series, or based upon self-filling magical reserves. If the former is chosen then there would need to be some way to limit the amount of magic one should be able to use, most likely via physical taxation as more of the magical energy is used. It is my personal preference that magic should be based off of a central source and users limited by inherent magical strength, having a predetermined magical endurance to which they can grow, most people having little to no such reserve with a minority being able to utilize magic.
With source out of the way we come to how magic should actually be utilized. As stated earlier I believe it should take a more free-form approach. This does not mean however that there aren't limitations. Magical 'spells' should, in my opinion, be complex or semi-complex weaves of different types of magical energy, specific combinations required for a desired result. Such learning would limit magic to whatever the user could form. One must also address strength. Too powerful and magic supplants technology, too weak and it is no longer important. It is of my opinion that the very highest end magical users, nigh-immortal creatures and similar beings, should have magical abilities that can affect situations on a planetary scale. This does not mean that magic cannot be used to affect a distant part of the galaxy rather that the maximum area of effect should be no larger than a planet a few times larger than Earth. In this way magic can still be used powerfully but in a localized area, making its use more specific in nature.
In short, magic should be drawn from a universal pool, each person having some sort of natural magical 'storage' of varying size, most undetectable. Magic should take a more free-form structure, users being able to utilize said magic to 'weave' spells that they have learned. Strength of said spells should never exceed a planetary effect.