Hmm...
Chandra x Gideon is pretty much a given, very good chemistry there.
Jace needs someone with strong ethics and personal initiative, as well as a sort of helpfulness who will push him to grow and do his best, I feel like he shined the most as an underling to Tezzeret, who would ride herd on him and keep him from slacking off and taking things the easy way, perhaps Avacyn with Sorin as a distrusting father-in-law who is annoyed and Jace dating his precious daughter?
Lilliana x Sorin, they both need an immortal partner who isn't bothered by their use of dark magic, while Liliana needs someone who can remind her that black isn't all evil and of less manipulative options for dealing with situations, and Sorin needs someone who will challenge him on a personal level and encourage him to do more than just drift around occasionally trying to fix things and indulge himself, and get him to explain things more often and be somewhat more social
Kiora x Elspeth, Kiora's quest against the Eldrazi and drive to grow in power would be helpful for motivating Elspeth, and helping her recover from Pherexian related traumas, while Elspeth could help Kiora become a bit more grounded and respectful of others, and become more social, and Kiora could help Elspeth learn better grace under pressure and to better ignore incorrect opinions of others about her, and stop being so fatalistic. Also, Elspeth didn't show much interest I can remember in Koth or Venser, so the idea of her 'batting for the other team' makes sense to me, especially if greek divinities/culture equivalents were involved in her training at some point, and Kiora totally has a little-girl crush on Thassa and is squirming with pride whenever someone mistakes her for Thassa, or the only reason she hasn't already been hit with a smite by Thassa is because they're already doing it on the side.
Karn x pseudo-female!Karn - Karn could totally make his own girlfriend
New Pherexia x the Borg - a match made in a place that has to be worse than Grixis
Domri x Garruk - yeah, I wouldn't be surprised by this, not till Domri gets much older though
Ral Zerek x Tamiyo - they'll do sweet, sweet
SCIENCE! magic research together
Tezzeret x Vraska - Vraska will keep him fascinated and encourage him to rebel against and escape the influence of Bolas, as well as hone his sense of retribution to more controlled and focused of a form, and he'll teach her the value of organization and connections and working together, also, those dreads totally say he'd find snake-hair cool
Sharkan x Niv-Mizzet - Niv would show Sharkan that dragons can be about more than just viciousness and power, but are actually beings of great intellect capable of works beneficial to others, and would be a much better master for Sharkan than Bolas
Bolas x Chimney Imp - a cute little minion for Bolas to lord his superiority over
Ajani x Sharuum the Hegemon - she needs someone to help her get over her late husband, and he needs someone to teach him the beauty of civilization, and together they can really help New Alara
Nissa x Spock - our best hope of stopping the Eldrazi and the Borg/NewPherexia alliance, well, them and a
deflector dish.
Oh, and forgot to put one for me... hmm... probably Elspeth or Tamiyo... or both as a threesome!