I realise i'm half a year late for the discussion. Apologies for the thread necromancy, just offering some alternative ideas (in point form):
1. Set up an XP gain penalty per artifact a hero is carrying (assuming this can be scripted easily).
1.1 No penalty for 1 artifact in inventory, then some % penalty every subsequent one (i.e. 0% for 1st artifact, 20% every extra, for no XP gain with 6 artifacts).
1.2 Or, small penalty per artifact, including first one. Maybe 15% each, for max of 90%?
2. Apply penalties to heroes carrying artifacts
2.1 Passive penalties to some combination of stats/damage/armor/movement speed/attack speed/HP/MP/HP regen/MP regen, or maybe spell damage penalty? Unique for each artifact.
2.2 Alternatively, apply penalties after the artifact is activated (and the effects of the activation wears off).
2.3 One idea is to let artifacts upgrade themselves as the hero levels up, but adds/increases some penalty.
3. Random artifact purchase at artifact shop. Goods sold not returnable
3.1 Maybe make them 'cursed' (undroppable) as well.
3.1.1 Needless to say, artifacts may have to be cheaper as compensation...
3.1.2 This may pose a problem with upgrading
4. Put in a Gold (or crystal
) cost with each activation (For Orb of Fury, that would mean the shockwaves...).
5. Require a skill point to carry an artifact, +1 for every level of upgrade.
5.1 This would mean that a lot of crystals would have to be spent on buying skill points... Maybe this can be combined with the 'cursed' idea in 3.1.
Combinations of these (along with the other suggestions in the thread) may be viable too.
That's all for now.