Could you say in your backstory that you became a reborn in your past so that you could get all the goodies of both classes? I am seriously considering that to get a flying speed and advantage on death saving throws. 

I assume by "classes" you meant "races".

If the DM allows it, then yeah you can be a reborn with an ancestral legacy from a previous race.

Reborn, Hexblood, and Dhamphir are all "legacy" extensions which can be added to any race....if your DM allows it. They can be stand alone races as well, and were designed for existing in Borovia. They definitely unbalance the racial strength especially if you are choosing a race which is already a bit overpowered.

I was rubbing my hands thinking I could be a reborn V human and then figure out I don't get a feat

Reborn and the other lineages already have the Ancestral Legacy trait, which incorporates parts of a previous race into your character. So, no, the lineages aren’t necessarily unbalanced, as Chris suggests.

They become more unbalanced when you take them out of their respective worlds, IMO. The Ancestral Legacy is Ravenloft specific and aids in surviving in that world. When you begin mixing "rules" designed around specific world designs, it can most certainly lead to specific character imbalances. It all depends on how your DM feels your idea will fit into their world and relate to the other characters in the end. Playing a Reborn Tabaxi with a Strixhaven/Revnica background and use the MotM character options to have greater freedom in choice will make a significantly more powerful base character before even picking up a class over someone that might not know about those options if they are allowed to use all available published materials.

So...just ask your DM if they are cool with it before settling on using it as a concept. It looks like you are focused on a flying race like an aarakocra, fairy, winged tiefling, or owlin I am assuming, so ensure that which ever one you choose does not lose something you want to keep.