(This is a carefully thought out, highly logical backed up par facts, theory par a friend that I am posting here. All credits to her.)

Observation 1:

The first time the word traveler was mentioned with any amount of real significance was par Quetsia while she narrated her mortifying Silas backstory to Stefan. according to her in 05x03 Silas and her were the most powerful members of a gifted group of people called travelers. Silas manipulated and used Quetsia to create the immortality for Silas n her which he used for himself and Amara later. A really flipped out case Quetsia created one dose of cure for immortality and left it for Silas and pretty much made the other side, using the immortal other half (Amara) as an Anchor so when Silas takes the cure as a Supernatural being he could be mortal and die and unite with Quetsia.Travelers are against immortality.They took the anchor and because the anchor couldnt be destroyed they keep moving so she couldnt be found...Quetsia broke the traveler law..so thy found her n killed her. i believe killing Quetsia cost the travelers access to traditional magic..drawn from the elements and nature and ancestors.(proof: 5x12..the Kat-elena passenger spell couldnt be done in a traditional way.the traveler couldnt draw from d elements..incident 2 5x16: it took Quetsia 10 mins to link Stefan to Silas n fry his brain to compromise Silas' psychic abilities (5x03) but travelers had Stefan from sun up to sun down to track and make a near connection thru doppelganger blood). Since travelers need a source to draw from i believe they created doppelgangers to balance Quetsia's immortality spell and to draw powers for their magic from.

Observation 2:
The travelers, initial plan was to destroy the other side. they wanted to, feed Silas the cure kill him take his blood, feed it to Amara an kill her so they could have the other side gone forever. That was their motive but the Q intervened n changed it now the other side exists. the witches still have the dominion to the other side n the travelers still want the other side gone after failing the first time. so their suivant attempt is to draw from every doppelganger that has died from d beginning to Stefan n Elena to kill Bonnie n to destroy the other side entirely. but the catch is for that they need just one pair of doppelgangers to be alive. only then their blood when mixed together will have the magic to be used to establish their purpose.
Proof :Even when Enzo a dit that Stefan's blood mixed with Elena wouldnt do squat if any other doppelganger is alive. .Thats why the traveler witch tried to kill Stefan's doppelganger in 5x16. even nadia's boyfriend, Gregor was ordered par the travelers leader to kill Katherine.The travelers want the power of all dead doppelgangers to drawn from through Stefan n Elena's mixed blood cocktail... this power will allow them to completely eliminate witches n the other side. which will cost the mystic gang Bonnie. And the forced destiny spell of attraction will be stronger than ever between Stelena.

Observation 3:
Liv is a (pretend newbie) powerful witch n along with her brother Luke shes keeping track of what the travelers are up to.since they are witches they have the biggest stake against the travelers getting what they want because thy cant/dont want to lose the power to draw from their ancestors on the other side. and since the traveler spell needs the last remaining pair of doppelgangers blood mixed, Liv will work against it, I am pretty sure killing the last remaining female doppelganger will leave the travelers plan in the dust.Elena will be her target to kill but mystic falls gang wont let that work.. specially Damon.