Could be that at Eneas Lobby, there is one or more Poneglyph Stones that Nico has yet to see? it might explain Nico's motives (fulfil her dreams, her stubborness, etc).
imo it is possible for the Marines or WG might have collected one or two poneglyph stones for their own over the many years and put them at Eneas Lobby. anyway, it can't be that all Stones are scattered in lost civilisations and hidden in secret rooms. i think it's fair that WG possesses at least one of em…
And so Nico wishes to read all the Poneglyphs in order to discover the True History. She was probably told from CP9ers that there is one at Eneas Lobby, but cannot reach it w/o giving herself to the Marines/WG, AND she doesn't want to include Strawhats in her dilemma. so she half-heartedly goes into all those troubles to keep the crew's safety...
so far, there was a poneglyph stone each at the end of the 2 previous major arcs. y not want at Eneas Lobby, unless the major arc has still a long long way to go.....but i think it's about time another one pops out.
waddayathink? just my thoughts.