What is the "timeline remnant" concept from The Flash S02E11 , which somehow accounted for the return of Eobard Thawne (Reverse Flash)?

The most logical and correct ( presumeably) answer to the concept of timeline remnant is a situation of a person who has two different future version because the chronological time got fractured for that particular person due to time travel. Also, there are other things to remember as have been said by wells (from earth-2) that a timeline remnants are possible in case of speedsters as they are protected by the effects of time travel due to the speed force in their systems.

Taking a simple example:

In case of eobard he traveled back to the time to kill barry's mother, so basically that means he traveled from 2140 to 2000 to kill barry's mother and got stock. Now for this eobard, his past will be 2140 and future would be the 2000 ( which is past for us), now he would grow older and then eventually be killed because of eddie's death. However there's already another eobard in future let's say in 2100 who can always travel back in time to let's say 2025, so this eobard can be called as timeline remnant when he traveled back to 2025, as he has no future nor past.

Merging with his alternate timeline self seems to be most likely, as this is what happens in the comic Flashpoint. In the comic, Barry wakes up to find a world changed in the wake of his time travel, and crucially — he has no powers.