Possible Conversions
It depends a bit on what version you've got: I did some conversions from RuneQuest II to first edition AD&D, but that was a long time ago - and I no longer remember exactly what options were from which version of Runequest. For current conversions, as I recall, the Sorcery Skills in later editions are a fair match for the build-a-spell systems like Theurgy or Dweomer, the totemistic Spirit Magic from the Hero Wars version fits fairly well with Hexcraft using the fixed slot options, most cult runepriest runespells (bought with power) could be taken as Innate Spells at a reduced cost ("Specialized") to account for the very slow regaining of abilities, most basic spirit magic would fit as mana-powered innate spells or rune magic (most of these options are from Eclipse: The Codex Persona, since that's what we use for our d20 games. The shareware version is linked in the sig). The framework would probably have large hit dice at the lower levels, small ones at higher levels, a heavy emphasis on defensive abilities (to raise AC and saves) at high levels, and fairly modest magic. For "straight" d20 - well, treat all spells as one big list, give everyone 4 bonus levels of Favored Soul but require that they be trained to actually learn spells, and forbid all other spellcasting. Initiates of particular cults may take additional levels of Favored Soul, up to a maximum of an additional eight levels in it, but only regain the additional spells they acquire in this way on a weekly, rather than on a daily, basis. Levels 5, 10, 15, and 20 automatically go to "buying off" the initial bonus levels. This makes "low-level" characters quite competent in physical combat, gives everyone access to a limited amount of minor magic as they learn it, and still keeps major magic rare. At higher levels it will begin to cancel out, and at the highest levels the advantage will vanish completely.
I might also cap the campaign at about L12 or L16 for classic Runequest, but for Hero Wars the skies the limit.
