You don't even need a good excuse for it. Have Zavala say something about how Osiris and Rasputin are being more cooperative, and want to run simulations of your most dangerous opponents (Atheon/Crota/Skolas/Oryx/Aksis) to both see what happens and what might happen if different modifiers are applied (Osiris' stake) and to add it to his predictive algorithm (Rasputin). From here, you can access any of the four raids and maybe Prison of Elders through the Infinite Forest, which are just straight ported from D1 with some slight adjustments to take into account new mechanics from D2. Each week has a featured raid with prestige modifiers in it, which are just stuff like Glass or Blackout or Heavyweight if they want to be easy, but it gives you better drops, such as doing the featured version of Vault of Glass gives you a chance at Mythoclast. The Forest exists, Bungo, use it.