What if future lucy is Zeref in disguise? His obcession with Natsu could be behind his rouse. And he might simply be using the whole eclipse thing not to safe the country but to destroy it. Perhaps its all just a rouse on his part to fool everyone into activating eclipse. This would explain why Jellal felt something reminiscient of Zeref in future lucy.
One thing that might also be considered is that future lucy may have used a magic similar to Zeref's or of the same nature to time travel. This would explain those "traces" similar to Zeref.
Lets not forget, Jellal never met Zeref. He was never on the same physical space as Zeref. He senses magic that is somewaht reminiscient of Zeref's magic, which he encountered in the tower but also in demons around the world. Zeref used a long forgotten, lost magic unique to him through which he created thousands of demons to destroy the world and would have reportedly used that same power to transition acnologia from dragon slayer to dragon.
If Zeref is future lucy, returning to the grand magic games year after year might be to search for natsu. In the hopes that he can see natsu which he believes to be the key to his own salvation. It would make sense. The crying upon seeing Natsu. The double nature of the situation in itself (Lucy comes back from the future to build a device that would allow her in the future to time travel back...yet she hasn't don anything to change this hellish future in 7 years...Its like a closed circle without a solution or much logic) would play into the double nature we have seen in Zeref. In one side he's a fragile sad depressive kid who cries as he hopes to be saved from all the evil he did. On the other side, he's a saddistic evil mage who once plnuged the world in darkness and even help give birth to the biggest enemy the human kind has ever faced. This dual, uncertain, illogical, inconsistent nature would fit with the current state of things.
