The utility / power of external synnergy is also affected by whether or not the external source is available under the same element.
E.g. 'off-element' is external interaction which tends to further weaken the availability of the synnergy (in terms of deck design) as opposed to 'in-element' external effects which tend to be easier to accomodate in deck construction.
I think cardinal elements (
) are probably the next most obvious choice after material elements since cardinal elements all deal with the dimensions / mediums in which things interact (e.g. time, space, order / disorder) and distortions thereof.
These seem like very good candidates for effects that directly manipulate the position of creatures in the field, hand, and deck while the material elements seem good like good candidates for introduce positionally dependent effects.
I think spirit elements may shine for having creature centric field-effects. E.g. effects that cause a creature to influence the creatures near to it (although material elements could certainly have claim... e.g. proximity damage from fire or frost)