It's funny, OOO wouldn't be in my top 10 Rider series nor was Ankh my favorite character from that show (Date FTW!) yet him coming back and Eiji transforming into OOO was a highlight and I was sad Ankh didn't get to stick around.
Also that bike-based action scene might be the longest some of them have ever had.
The only two nitpicks I have include it resolving nothing with Foundation X...
It's very much on the record that I don't care for Foundation X so I didn't mind that they were completely glossed over.

Though admittedly it makes Foundation X's appearance in the Brave special make no sense.
Although, I'm a little confused as I thought Ghost, Ex-Aid, and Gaim were all in their own universes.
Ex-Aid appeared in Ghost's final episode and all three were in the previous "Heisei Generations" movie. In this movie Takeru (Ghost) mentions wanting to repay Emu (Ex-Aid) for saving his life, which refers to the last scene of that film where Takeru has a heart attack for seemingly no reason and Emu had to resuscitate him.
I think the parallel world stuff was there just so they didn't have to explain why the Sky Wall wasn't in the previous Rider series even though Build insists it's been around for 10 years. No doubt in future crossover films Build will appear without explanation.
Oh, and who was the dude cospalying as Brooke from One Piece? Is that the second rider from Ghost?
*Googles who 'Brooke from One Piece' is*
Oh, you mean Onari. No, he's not a Rider. He was the bald Buddhist monk who acted as the comic relief character in Ghost.
At the end of the series the writers thought it would be funny for Takeru to declare that ex-villain Javert was the new master of his dad's temple (seemingly just to screw Onari over for the lulz). In the post-season V-Cinema 'Kamen Rider Specter' it's shown that Onari has quit and is working as a detective, and for some reason is dressed like that (I assume it's a reference to something I don't understand).
So the outfit (along with his reconciliation with Takeru at the end) fits with continuity but I suspect the real reason he's wearing it is that the actor has probably grown his hair back since Ghost finished filming and the wig from this outfit disguises that.