I'll be keeping with the ley lines ideas. However there are a couple of changes to the rules and usual Fantasy Hero magic systems that should be noted.
Standard FH magic has gestures, incantations, usually an OAF expendable as well - and then you have a skill roll to activate the spell. Some spells have side effects if they aren't cast directly. In the Turakian Age published setting for FH, there is a cute school of magic known as Theurgy, which borrows power from the Gods or the Stars without their permission (priests of Gods have their blessings already, of course). The side effects of failing the Theurgy skill roll means the God in question has noticed and usually inflicted the caster with a lesser version of what they were trying to do.
Be that as it may - I can't have side effects in WS if you fail a skill roll. As I won't be having skill rolls to cast spells. The FH book suggests there are problems, as this treats spells as inherent powers. However - while I won't have a roll to cast a spell, I will require a roll to tap a ley line (for magic users) or to confirm their faith in their God (for priests) in order to power the spell. Mind you, a spell working is not the same as hitting a target - that still needs to be rolled as an attack, like any other weapon.
Strangely enough, the -1 to the skill roll that normal FH spells get for every 10 active points, also equates to how much endurance (END) is needed to power the spell (unless it has modifiers to the END). And END is what they ley lines provide to power spells in my WS setting - spell/miracle users don't use their own endurance (unless there is very special circumstances). This leads to the interesting concept of skill roll equating to effectiveness. And failing a roll is less catastrophic.
For most FH or Turakian Age - you roll skill, if you fail you get side effects, or nothing happens.
In WS - the spell goes off, you roll tap skill, if you succeed it is at full effect, and for each point you fail the spell is 1 END less effective. 1 END of power usually equated to 1D6 of effect.
The question is - what to do with people who succeed their tapping roll by a larger amount than what is required?
Spells are a set cost with a set effect (so many D6 and no more). Should I have a variable advantage on all spells - so if the caster manages to get even more END to pump their spells with, they could add extra advantages to it - such as armour piercing, explosive, area effect, auto fire, modifiers to hit etc..
Possibly for every +2 they made their roll by, then get 1 END worth of advantages - on the basis that some of the energy is wasted as they struggle to shove it into the spell they are using.
If I make this a house rule - I won't need to modify any of the spells in any of the books. This is just the way that they work in my setting.
However - I'm still not sure what to do with side effects that occur only on a failed roll, it is now a lesser disadvantage. I'll see what spells players choose and do a case-by-case analysis I guess...
I've an idea for simplifications for this- if they fail their roll, they get the "weaker option" on the spell (as listed in FHG, FHG2), and if they are a magic user, and succeed with a control roll before a tap roll, and the tap roll succeed by 2 or more points, they get the "stronger spell option". I'm also thinking of having a second level of failure.
Spells from TA, FHG, FHG2
Priests use Faith
Secular use Tap
Faith/Tap roll modified by Active Points of spell (as normal), Magical Weather (as normal) and Magical items in the vicinity (MIV)
Secular with Tap-Ranged (an advantage to Tap) can bypass the MIV and Magical Weather modifiers.
Secular with Magical Control must roll it before Tap, or Tap-Ranged.
Anything that affects concentration, as other skills. Also long-term endurance loss/fatigue, and a minus for each spell currently in use.
NB- because the player knows when they fail an MC roll, it adds an extra level of choice, do they then still cast the spell knowing it will be as uncontrolable as a Priest's miracle?
Active (attacking usually) power -
Every 10 active points -1 (of currently active power)
Passive (defensive usually) power -
Every 20 active points -1 (of currently active power)
Alien magic (ie Demons) -5 (the alien magic is polluting the local ley lines)
Another thing I just realised, Tap-Ranged will have to include modifiers for range based on the next nearest ley-line. Which could mean that it is actually a worse roll than one affected by weather and MIV. However, it is cheaper to buy skill levels that "affect range modifiers only". So that should counter it.