To answer your question, the sixth season concepts for "The Batman" included the introduction of Hush, Scarecrow, Two-Face, Ra's Al Ghul, and there was even talk of lesser known villains like Copperhead and possibly even Crazy Quilt being included as new villains, while additionally there was mention of The Creeper and Etrigan getting cameo episodes.
Aaaaaannnd then the horrific All-White-Male Testosterone Club fifth season of TB ruined everything and all plans for the 6th season were cancelled and put into the filing cabinet of Things Never To Be Concluded at DC-Warner Bros. Just like the rest of season 3 and all of season 4 of Batman Beyond. ;(
I don't know where all of the concept art was, or how much was done, given a lot of what I was told was given to me by word of mouth, but I can show you the concept art for Scarecrow and Two-Face. 25.media.tumblr.com/tumblr_lx9… www.worldsfinestonline.com/WF/… And if I'm not mistaken, Etrigan at least, made it into an issue of "The Batman Strikes!"
Yes, I'm referring to the Justice League of Season 5. Their stories pretty much dominated the season, and it was rather a buzzkill that they chose to leave out Wonder Woman when they even made concept art for her. www.worldsfinestonline.com/WF/…
But of course.
If I recall correctly, Two-Face and Scarecrow were going to be the poster-villains that were going to be advertised early in the sixth season. Like how they made a big deal of announcing Croc and Riddler in season two, only so they could then sneak in Hugo Strange. They picked two popular villains for their ads, to draw people in, then revealed a third who was supposed to keep the fans solidly hooked. Two-Face and Scarecrow were the two they were planning to advertise and draw people in, so they could hint at Ra's in a later episode and then they would finish up a two-part season finale with a big battle against Ra's.
Come to think of it, they mentioned also that Dracula in the "Batman vs Dracula" movie was their original super-early test model for a design variant for Ra's in hopes they might be able to introduce him later. They weren't sure whether or not they were going to use it, saying the flare of the collar made Ra's look too much like vampire, so they instead used it as their Dracula design. Later, while season 5 was in development, they were given the green-light for putting Ra's, Two-Face and Scarecrow into a season, but we all know how well that turned out, given season 5 pretty much tanked, statistically speaking.