It didn't really come across to me as like that. It came across far more like "Yoooooooo!!! Look at how humongously big testicled this Xebec bloke was!!!" which is weird if Xebec doesn't come back into the story somehow. I could accept that reasoning if Xebec was only the captain of Kaido and Linlin but he is also the captain of Whitebeard, Shiki and several other legendary dudes. That is completely overkill. Lol.
Whitebeard already had a short scene that humanized his past rookie self when he was on a different crew and talking about family.
The only other thing the Rocks gimmick did was form a basis for Kaido and Linlin working together ("Hey, we used to rock shit in the past. Let's do that again") but Oda could have just made them Shiki's subordinates (say his YC1/2 equivalents) instead of creating an entirely new mega, super, hyper, ultra pirate character that was so ridiculously powerful that nearly every notable pirate captain from one generation ago bar Roger was his bitch.
I might agree that it makes OP bigger if it wasn't for one thing.
This bloke is yet another Rocks stooge.
Having nearly every single, damn pirate be connected to Rocks just makes this world increasingly small to me.
Like I said, having so many notable characters be Xebec subordinates only serves to hype the guy himself and unless this guy is brought back from dead then I really don't see what the point is. Even then, he would be like a fourth final antagonist in this series when we already have Akainu, Blackbeard and Imu.
Also, petty talk speaking but his name completely sucks. "Xebec D. Rocks" sounds like something an A.I. random word generator would come up with. Lel.
Think its pretty much obvious at this point Rocks main place in the plot is related to events in God Valley that we are yet to know more about.
The introduction of Imu and Rocks in the story was close and after we discover that Rocks wanted to take Imu place as King of the Wolrd and have connection to "taboo knowladge", its a way to most likely confirm that Rocks knew about Imu, and pehaps even have some connection to him.
There's also two other plots that came from the God Valley:
1-Garp connection with the Celestials: Sengoku points that he protected the Celestial and their slaves there, think thats a hint on this connection, with the reason for he to protect them being more than just duty. With pehaps Dragon mom being a celestial, or most likely a slave of the Celestials and that was sacrificed by them in God Valley, that could be the trigger for Dragon to want to destroy the Celestials, as he would be 17 at the time.
2-Shanks past: Its hinted that Shanks was taken by the Roger Pirates as a baby, the film Red even hint on he being taken in God Valley and in his talk with WB, Newgate say that looking to Shanks face made the scars he got from "him" ache, at the time seems kind random, and mostly think he was talking about Roger, but today seems he was talking about Xebec and how Shanks look like his former captain, most likely because of the Red hair, that Xebec seems to have, at least in the anime. What would make Shanks real name Rocks D. Shanks.
I personally dont think Rocks will come back to the story as a living characteres, but his Will clearly still live and influece pirates like Teach, and to me thats will be his great "contribuition" to the plot.