Yeah, as much as I'd like to see Slayers on Toonami, it's most likely isn't going to happen. The first three series are too old and I'm not sure if they'd be interested in Revolution/Evolution-R. Revolution was pretty lackluster in my opinion, but Evolution-R was an improvement. I consider Revolution/Evolution-R to be one series, rather than two, and I would consider it an improvement over Try in most areas, even though I also didn't think Try was nearly as bad as I had first heard about it. Slayers Next is still the best Slayers series and it would be great to see it on Toonami, but it's sadly not going to happen. Too bad that the original three Slayers series didn't air on the original Toonami. That might have made it more likely for them to air Revolution/Evolution-R now at least.
In terms of more a light-hearted show, I think that Kenichi: The Mightiest Disciple would be a good fit for the block. It has a pretty good balance of action and comedy, most of the characters are likable and/or entertaining and it's a pretty fun series. The pacing can be a bit off, especially when the episode recaps can go on for five minutes, the fanservice is so blatant as they focus on nearly every female characters' breast and/or body all the time, especially during fights, and the ending, from what I remember, was pretty lackluster. Still, it was a pretty enjoyable series to watch with some really good fights and pretty good comedy, so I wouldn't mind if they eventually got that for Toonami. It would be a bit refreshing to have something a bit more light-hearted on the block.
Another possible show they could look into could be Shakugan no Shana. I'm not sure if it would be exactly light-hearted, especially given its initial premise and the world they setup, but I could kind of see it working on the block. Unlike something like Blue Exorcist, the first series actually has a good balance between the fantasy action and slice of life drama where neither side overpowers the other. That goes out the window for the first half of the second series though and it takes them way too long to kick start the plot, but its second half kind of makes up for that. Final is practically all action with little romance, which at that point was a good thing in my opinion considering how that brought down Second so much, and the storyline is better, but it suffers from being way too faithful to the source material when the other two series weren't. It throws in way too many new characters all at once and one of the main leads becomes incredibly unlikable to the point where I wanted someone to punch him all the time, even after they explained his motivation. Still, when the series is good, it's usually pretty darn good and I could see it fitting on Toonami pretty well.