It is possible to have a signature spell being accessed earlier... but I believe it is better to improve the effect, once they acquire it. Also, usually it was bards or paladins who got early access... with the improved progression, there is no need to reduce the level requirement for such spells. Most spell are at a certain level for a reason. And things like daylight in 3e were very confusing in their interaction with darkness:
clerics and wizards got it on different levels. Darkness countered light levels of level 2 or lower... ok does it counter daylight or not?
Those sort of things should be (but for some reason never are) clearly delineated in the write-up for each spell.
But (for a probably-inaccurate example) Druids getting Endure Cold/Heat at 1st level where no other caster gets it before 2nd just makes common sense. Also, Clerics (should) take longer to cast spells of a given level than Wizards, thus a Cleric casting Darkness (C-1) should take longer than a MU casting Darkness (MU-1) even if the spells are otherwise much the same. And so on.
It's not that hard to put all the differences in the same spell write-up. For example (using 6-segment rounds):
Darkness (Clr-1, MU-1, Ill-1, Drd-2, Brd-3)
Casting time: Clr 3 seg.; MU 1 seg.; Ill 1 seg.; Drd 3 seg.; Brd 1 round
Area of effect: 20' radius sphere
Duration: 1 round/level for all except Bard: while song maintained
Range: 100' line of sight except Bard: 100' line of sound
Saving throw: special, see write-up
Components: V-all; S-all; M none except Clr: holy symbol; Drd athelas leaf; Brd instrument
<insert spell write-up here including how it interacts with all sorts of other light and darkness spells and effects>
There. Isn't that more efficient than writing up 5 different spells (never mind trying to dream up 4 more names that mean "Darkness")?
Lan-"here in the dark"-efan
