I feel the series is hinting towards it's end with the mention of the 4 red pony glyphs and the events after that happened super fast.

I also think that the series has a good 5-7 maybe 10 years left in it. But it looks like it's forshadowing its closing end.

A few loopholes left are:

Will of D.
Raftel
4 Ponyglyphs
4 Yonko's
Void Century
World Government and CP0
Revolutionary Army
Black Beard

I can't think of much unless Oda decides to throw in other things arbitrarily like he always does.