Basically "True Prime Roja" will stay featless, his best feat forever being sharing the spotlight with Garp
We know from Toki that, at least in the form of a Fruit, they had time travel. Was that all the time travel they had? How Chrono Trigger are we getting, here? Did they have cryo technology? Vegapunk does... Could they have put themselves in statis, for the future?
Since Laugh Tale is decidedly connected to the Ancient Kingdom... I believe Laugh Tale HAS Ancient Kingdom remnants, and that is who is guarding it (I also believe Pluton and Zunesha have Ancient Kingdom remnants living inside of them, who have been unable to leave since they entered during the Void Century).
IMO, the top tiers Roger fought weren't WG/other Pirates... but Ancient Kingdom forces, guarding Joy Boy's treasure.
Like the Roger Pirates remained in convenient places to aid Roger's successor, I believe there are a good number of Ancient Kingdom remnants that were on Joy Boy's side, and are awaiting his successor. I also think there's an evil Ancient Kingdom faction kicking around somewhere via ancient tech like cryogenics and time travel. If they DID time travel, they would just... pop up whenever they set their time to, like the Samurai did in Wano.
We've see Roger with Shiki, with Newgate, with Rocks, and it doesn't seem like Imu and Roger met again, especially with Gyaban saying he's never felt Haki like this before, not even on God Valley.
I think the only group of strong fighters Roger could really go through to reach Laugh Tale, is the Ancient Kingdom themselves. It couldn't have been the Marines, because, well, then they'd know where fucking Laugh Tale is.
Roger's big fight was against Ancient Kingdom forces in Laugh Tale, this is where he proved he was worthy of Joy Boy's legacy, and could help pass it down, IMO.

