Oda basically already laid out the ending of OP, when he said that "the most exciting/adventurous part of OP will be over in 5 years"
He plans to make Luffy PK by then, which means Blackbeard will be defeated in that timespan.
Like you said, that doesn't mean OP is over though.
This entire story has been leading up to "the dawn", "the final war", "CDs vs the will of D" etc., which will happen after those 5 years.
Luffy will go up against Imu and the CDs. I bet that Imu will also be his final fight. The ultimate D vs the ultimate CD. The fight for the freedom of the world, with 900 years of history on Luffy's back.
It makes no sense that he will just fight someone like Akainu during the final war. Not only does it contribute absolutely nothing towards the "CD vs will of D" storyline, it's also just boring at this point. Luffy will have defeated prime-BB by then, Akainu really isn't much of a challenge after that.