It's possible that Baron can switch out his energy driver belt in battle, so he can use his Lemon Energy arms form.

Link to the official website
http://btw-sousei.bn-ent.net/sp/
And apparently the game is still one player only

So pretty much, they're reusing the same plot from Let's go Kamen Rider again...

Not exactly.

The whole premise of Let's Go Kamen Riders was something from the present, when left in the past, completely changes history. Here it looks like Riders are being erased one by one.

This game looks sweet, but how do you play it on an American console?

Playstation 3, 4, and Vita are all region free. Japanese games play without any problems. You can also get access to the Japanese PSN for DLC by making a Japanese PSN account (there's plenty of tutorials out there on this), but it can be a pain especially if it's paid DLC you're after (Battride War's DLC has been all free though).

I wonder if Chaser will be playable?

I am expecting Chaser to hopefully be some of the Free DLC along with Spectre.

The graphics don't look half bad, although these are cutscenes, so maybe it actually looks really really bad.

I guess having it on the PS4 could be why there was no BW game this year. Maybe Namco Bandai put their time on making the game on both the PS3 and PS4. Xenoverse on the PS3 and PS4 both look good despite being a cross gen game.