Seems to me the best route to get Erik and Raven in the same scene is to start with them separate, doing their own things, motivated by the things they are each motivated by (which are different).
I could see Mystque working to protect her mutant bretheren, as we've seen. I could also see her having formed her own team, that takes on whatever threat is there that is against mutants. If a government agency had a mutant, like say Destiny, captured then Mystique would most definitely try to free her.
For Erik, what is his motivation post DoFP? I could see him being an isolationist. But at the same time, I could see him wanting to protect hsi fellow mutants as well. I could see him forming the island nation of Genosha (could you imagine him actually using his powers to mine gold and paying for the island, all legal like?) or Asteroid M (Avalon) - or first one, then the other.
At the same time, I could see Mystique and her team fighting to free a mutant or mutants from a government agency, losing (or about to lose) and Erik saving the day. That would be a great, exciting scene and would end up bringing both Erik and Raven together, which sets them up to then have the "you left me"/"I had to" dialog, that would lead to the reveal that Mystique ended up either being tricked by Azazel, or forced, or that she sought comfort in his arms, all leading to Kurt.
What I don;t want to see is that Kurt is Erik's (and Mystique's) son. That would very muich ruin things for me. Makes much more sense that after Erik left, and didn't come back, that Mystique ended up with Azazel, and Kurt was conceived. Azazel was killed by Project Wide Awake, and they were coming after Raven, who fleed and left Kurt with the Gypsy Margali, which caused Kurt to grow up in the circus. This does two things...
It resolves the parentage of Nightcrawler, while also resolves the relationship that Mystique and Magneto had. Maybe they could get back together, but if Erik was leaving the planet (Asteroid M) I would imagine that Raven would feel the need to stay and continue fighting for the mutants. All of this would greatly expand on both Mystique's and Magneto's characters!