A spigot plugin adding "spells"/projectiles shot from the bow which target the entity you are looking at (almost). Download the plugin at https://syscy.de/downloads/SpellTest.jar

SpellTestPlugin.java

package de.syscy.spelltest;

import java.util.Collection;

import org.bukkit.Location;
import org.bukkit.Particle;
import org.bukkit.World;
import org.bukkit.entity.Entity;
import org.bukkit.entity.LivingEntity;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.EntityShootBowEvent;
import org.bukkit.plugin.java.JavaPlugin;
import org.bukkit.scheduler.BukkitRunnable;
import org.bukkit.util.Vector;

import lombok.RequiredArgsConstructor;

public class SpellTestPlugin extends JavaPlugin implements Listener {
	private static final double MAX_TARGET_DISTANCE = 100;
	private static final double TARGET_SEARCH_RADIUS = 10;

	@Override
	public void onEnable() {
		getServer().getPluginManager().registerEvents(this, this);
	}

	@EventHandler
	public void onBowShoot(EntityShootBowEvent event) {
		if(event.getEntity() instanceof Player) {
			Player shooter = (Player) event.getEntity();

			event.setCancelled(true);

			double targetSearchDistance = MAX_TARGET_DISTANCE * event.getForce();

			Vector playerDirection = shooter.getLocation().getDirection();
			playerDirection.normalize().multiply(targetSearchDistance);
			Location searchLocation = shooter.getLocation().clone().add(playerDirection.normalize().multiply(targetSearchDistance));

			Collection<Entity> targets = searchLocation.getWorld().getNearbyEntities(searchLocation, TARGET_SEARCH_RADIUS, TARGET_SEARCH_RADIUS, TARGET_SEARCH_RADIUS);
			LivingEntity finalTarget = null;

			for(Entity target : targets) {
				if(target instanceof LivingEntity) {
					finalTarget = (LivingEntity) target;

					break;
				}
			}

			getLogger().info("target: " + finalTarget);
			if(finalTarget != null) {
				SpellRunnable spellRunnable = new SpellRunnable(event.getEntity(), finalTarget, event.getForce());
				spellRunnable.runTaskTimer(this, 0, 1);
			}
		}
	}

	@RequiredArgsConstructor
	private static class SpellRunnable extends BukkitRunnable {
		private static final Vector TARGET_LOCATION_OFFSET = new Vector(0.5, 0.5, 0.5);

		private final World world;

		private final LivingEntity target;

		private final double speed;

		private final Vector start;
		private Vector end;

		private double time;

		public SpellRunnable(LivingEntity caster, LivingEntity target, float bowForce) {
			world = target.getWorld();

			this.target = target;

			start = caster.getEyeLocation().toVector();

			caster.getLocation().distanceSquared(target.getLocation());

			speed = 0.02 * Math.min(1.0, 1.0 - bowForce + 0.1);
		}

		@Override
		public void run() {
			if(time >= 1) {
				//				target.damage(2.0, caster);
				//				target.setFireTicks(200);
				end = target.getLocation().toVector().add(TARGET_LOCATION_OFFSET);
				world.createExplosion(end.getX(), end.getY(), end.getZ(), 10.0f, true, true);

				cancel();

				return;
			}

			end = target.getLocation().toVector().add(TARGET_LOCATION_OFFSET);
			Vector currentLocation = findPointOnLineBetweenPoints(start, end, time);

			world.spawnParticle(Particle.FLAME, currentLocation.toLocation(world), 100, 0.1, 0.1, 0.1);

			time += speed;
		}

		private Vector findPointOnLineBetweenPoints(Vector p1, Vector p2, double t) {
			double x = (1.0 - t) * p1.getX() + t * p2.getX();
			double y = (1.0 - t) * p1.getY() + t * p2.getY();
			double z = (1.0 - t) * p1.getZ() + t * p2.getZ();

			return new Vector(x, y, z);
		}
	}
}