- The arrows will have properties such as different/additional/improved criticals, effects like a gust of wind, be immune to deflections, make a sound as a signal, 'cast' an AOE dispel at the impact point, etc.
Thinking about this more and looking through Spell Law (don't have all of my companions with me right now) I'm finding very, very few spells that would work "on impact" or carry to affect a target.In RM2 all of the Essence Dispell [realm] and Un-[realm] spells require concentration, and Quiet, while it can be cast on a moving target (an arrow) only affects a 1' radius around the target. This basically reduces the arrow to a wand, that the archer would use when they nock it.
The absolute first thing I'd say is, if you're an RM user and you're going strictly by the published rules, you're in a pretty small minority
. They're magic. Make them work how you want. Still, the RMSS Treasure Companion provides the rules if you need them for various effects.
You could embed things like...
RMSS Closed Essence's Dispellling Ways you have:
4. Dispel Essence I — Any active spell from the realm of Essence
that is on the target must make are RR (use the level of that
spell’s caster as the spells level). If the spell fails its RR, it is
dispelled. (with Dispell Mentalism and Dispell Channeling at 5th and 6th level)
Un-[Realm] spells are against a target and lasts for a full day.
17. Unessence — Target has no Essence power points (and thus
can throw no Essence spells) for 24 hours (this includes spells
cast using spell bonus items). This can also be cast against items
which could normally cast spells. Hybrid spell casters who have
Essence as one of their realms only have access to half their
normal power points (again, the other realms at the next two levels).
In Open Channeling's Sounds Way you have the 7th level spell 'Silence I' with a 1min/level duration;
Creates a 1'R area around the target, into and out of which sound cannot travel; if the point is on a mobile object or being, it will move with the object/being.
This would prohibit, or at least provide a penalty, to spells with a verbal component. You cast it on the arrow and shoot them with the arrow. They won't get an RR as a result of the spell being on an object and not them, but they could pull the arrow out (possibly taking some damage/crit as a result).
Close Mentalism you could use these to destroy a wall for example...
10. Cracks Call — Any previous cracks or flaws in material up to 10'x10'x10' section will extend to their limit.
* I like the idea I mentioned above about arrow bonuses being able to modify crits. The fact you might miss, wasting the bonus, counters the power of crit-adjustments. You could also say that the bonus is halved to get the range of potential crit adjustment (e.g., +5 arrow can move a crit up or down 3 points, +10 can modify crit by 5, etc.)
Sounds like you're using RM2, so your options may be more limited if trying to use strictly published material/rules, but you seem to be liking the idea of what I suggest above. Make your own effects up. You could have all kinds of magic arrows with a little creativity.