Good question! In what I've learned 7th house personal planet (sun/moon/mercury/venus) placements are good, giving a partner a feeling of completeness. Jupiter in 7th is amazing for marriage prospects, Saturn in 7th marks it being a serious relationship as well and Neptune in 7th creates a special bond of patience with the other (this is in terms of non personal/generational planets).
Vertex aspects are fantastic as well! Our Vertex is like a second Descendant, and 7th house is our Descendant. So any positive contacts to our Vertex is considered "past life connections". Vertex touching the sun, moon, jupiter, etc in positive aspects are a very good sign.
Aspects to 4th house (IC) and South Node are good as well. 4th house can be a bit love/hate, as it represents family and home, and I mean- we all wanna kill our family sometimes. But there's that bond there, with Venus and Jupiter or moon in 4th house there's familiarity, a need to build a home together, etc. However, some planets like Neptune could be potentially problematic. As Neptune can turn malefic in houses, Neptune in 4th could mean the Neptune person is too erratic for the house person and may not be able to share a relationship with them.
And as for south node conjunctions, again, there's a wonderful familiarity there. Our south node is what we know, and when we meet someone conjunct to our south node you feel a sense of "picking up where you left off"
Honorable mentions to: Positive 1st house planet overlays (they support our growth and help us on our life path), Ascendant conjunct North Node synastry (instant attraction and themes like 1st house planet overlays), 2nd house placements (they will always add value to our lives and
our self worth and can even help us with how we handle our monetary issues)
