Here are some theories that could be how the modern-day scenes will look like:
Assassin's Creed: Infinity's prologue: I believe that Assassin's Creed: Mirage will be the prologue of Assassin's Creed: Infinity in a few words the Helix project and Basim would be a new memory that was added alongside the memories of the Sengoku Ninja and Wei Yu, the opening and ending scene will be like if we are playing a Helix-game and we are going to see in the memory menu, the Hidden Ones like Aya, Eivor, or Li E alongside other brand-new characters that could be featured in Assassin's Creed: Red and Assassin's Creed: Hexe.
The "New" Desmond Miles: The Animus user in Assassin's Creed: Mirage would be part of the Assassins or the Templars but he or she couldn't be on both sides, another theory would be that the user could be Basim's descendant who is testing a new version of the Helix app or Layla's relative and at the end he or she could join the Assassins or the Templars.
The Guinea Pig: Perhaps there is going to be a Desmond Miles or Callum Lynch-like character who was kidnapped by Abstergo to test a brand new Animus.
What do you think?