The stated goals of the Andromeda Initiative are to not only explore and establish permanent settlements on the "Golden Worlds" of the Heleus Cluster, but also to establish a reliable route between these colonies and those in the Milky Way Galaxy. So will the Nexus and/or Arks be sent on another 600 year-long integalactic journey back, to report the Andromeda Initiative's success to the Citadel Council? (assuming Shepard didn't refuse the Catalyst's options and the Citadel races succeeded in ending the cycle of the Reapers' "Harvest") Or will the Andromeda Initiative construct its own mass relay to connect the Heleus Cluster to the Milky Way's relay network? (not implausible, as the protheans constructed their own mass relay, the Conduit, before being harvested by the Reapers)

Personally, my theory is that after about 50-60 years of exploration and colonization, the Andromeda Initiative will set about establishing a practical means of travel back to the Milky Way Galaxy. I think this is why the Nexus was brought along as a "smaller, streamlined Citadel": like the Citadel, it is capable of functioning as a mass relay (under the right conditions). Remember from the first Mass Effect: the Citadel is a mass relay so powerful that it is able to draw on dark energy to open a gateway directly to the Reapers in dark space (the Mass Effect name for the intergalactic void). While we're never told how many light-years away from the Citadel that is, couldn't two Citadel relays with that much power succeed in establishing a connection across the vastness of dark space? This would probably involve taking the Nexus's ODSY drive system (which is probably such an enormous mass effect drive core already, in order to move such a space station across dark space) and somehow converting it into the heart of a mass relay. It somewhat explains the design of the Nexus as well, which does seem to mimic the pincer/tweezer design of the mass relays in the Milky Way Galaxy. So around 2841 CE, the Andromeda Initiative would be able to arrive at the Citadel, confirming that a new mass relay has been added to the Milk Way relay network.

I believe this is how BioWare intends to re-introduce Milky Way races not on the original trip to the Andromeda Galaxy. That post-game DLC will show the Nexus relay being activated, and Ryder returning to the Milky Way galaxy almost 700 years after he/she left. In addition, I hope this means that we get to meet certain long-lived characters from the original trilogy: Liara (now a Matriarch) and krogan like Urdnot Wrex and Urdnot Grunt (assuming that they haven't yet been killed by rival krogan). I'm particularly excited by the prospect of a Matriarch Liara, and the idea of seeing the Milky Way Galaxy almost a millennium after "The Shepard" saved it from the Reapers.

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