Training arcs are usually done to prolong mangas.In fact,they don't even make power ups more believable.
After tartarus,the brakes are off.This roller-cost won't be stopping.I imagine we are soon going to get more information about the plot...about what happened 400 years ago...about Acnologia,Zeref,Igneel...and then about Mavid and the heartfilia.
Zeref has the book now...and I believe he is capable of unleashing him.
Natsu will probably be going after Zeref,to grant Igneel's last wish for the book to be retrieved...and will then go after Acnologia to avenge Igneel.
Wendy can use Milky Way to manifest Igneel's spirit.He will be able to tell them what he knows...or what he wants Natsu to know.