There is already this question clarifying that you cannot cast spells with a range of self on the Shield Guardian, as you need to be able to target the guardian with the spell. It is clear that you can cast a spell that targets a creature "on" the guardian, as the guardian is a creature.
But what can then be the target of that spell, when the Guardian activates it? The description of the Spell Storing ability says:
the wearer must cast the spell on the guardian. The spell has no effect but is stored within the guardian. When commanded to do so by the wearer or when a situation arises that was predefined by the spellcaster, the guardian casts the stored spell with any parameters set by the original caster
I would take the target to be one of those parameters of the spell, and I am forced to target the Shield Guardian with the spell to be able to store it. Does that mean that it can only store spells targeting a creature, and those spells then effectively will have to target only the guardian when released? Or can there be a second, different target set for the spell when storing it?
This Q&A (which is confusingly titled to not ask anything about the spells storing aspects) indicates that any parameters not set by the caster can be chosen freely by the Shield Guardian, but is the target one of them? This Q&A states they plan to store Greater Invisibility into the guardian "with the intent to cast it on the party rogue", which would imply you can set another creature as the target, but the answer does not address this, either.
The second part of the question is: can you store a spell that targets a point in space? If the caster selects a point in space "on" the guardian, for example as the target of Fireball, could they cast Fireball into the Shield Guardian? And if so, the same question as above, can they set a separate point in space for when the spell is released or leave that up to the guardian, or will the Fireball have to be centered on the Shield Guardian?
Lastly, as a very specific case, if you can select a point in space, and can set (or omit) new targets, can you store Dimension Door into the guardian? The spell has a range of 500 feet (targeting a point in space), not Self, but it says "You teleport yourself (...)". Would it then teleport the guardian, who could possibly elect to take one other creature with it? (This would be pretty sweet, as my caster cannot Dimension Door himself and take the guardian along, as the guardian is large).