actually it seems that the way I thought it should work was already planned, here excerpt from SR4A, pg. 183
Some spells simply require a Success Test, with hits determining the level
of success (as noted in the spell description). The Spellcasting + Magic
test must generate at least one net hit to succeed and may need more if
the effect has a threshold for success. The spellcaster can always choose
to use less than the total number of hits rolled in a Spellcasting Test.
Spells cast on living or magic targets are often resisted, and an
Opposed Test is required. For area spells, the magician rolls only once,
and each target resists the spell separately.
The target resists physical
spells with Body and mana spells with Willpower. If the target is also
protected by Counterspelling (p. 185), she may add Counterspelling
dice to this resistance test. This Opposing dice pool is further modified
by any positive cover modifier the target might benefit from (see
Defender/Target has Partial Cover or Good Cover, p. 160). If the target
of a spell is on the other side of a mana barrier (p. 194), dice equal to
the Force of that barrier are added to the target’s resistance test. The
caster must generate at least 1 net hit on the Opposed Test for the
spell to succeed.so while partial cover/good cover are covered
, and visibility is in the SM pages, the only non-canon things, that IMO make sense is that if we treat spellcasting as ranged form of astral combat all other modifiers like freinds in melee, higher ground, spellcasting while in melee could be applied.
