You might not always get what you like :3 I'm going to bed now, but I'll drop a couple opinions to help get things rolling.
For PPGD, I find it strange when someone dies. There's a decent build up before it each death, so I wouldn't want to see Bell die. My one and only real reason, I don't think it'd be done well.
As for the other villains listed, I'm more indifferent about them really. Having them die, just for the sake of death, would make the series feel less like a story. The main antagonists need a bit more time before dying, tossing them away for no reason would just seem like such a waste. I feel the same for most of the protagonists as well.
If I had to choose one from PPGD, I'd go with a less popular character. One that most would know and remember, but wouldn't want to see more of. Probably someone from the PPG's past. I'm too tired to try and think of a specific, but the comic is lacking villains from PPG.
Honestly, I'd want to see a death in GT. Specifically, Mimi. I just think it'd be a brilliant twist if Him grew tired of Mimi then figured that she'd one day seek vengeance, and he decided to use her one last way. Being, to throw Mimi at Mandy, knowing that Mandy would likely kill her. This gives Him a reason to instigate a war, while taking care of a future problem.
At first, I though Chi's introduction made this idea less appealing; however, now I believe it's even more desirable. Mimi's death would bring out the development Chi needs, and it has the potential to be great. Aku would need to better contain Chi, especially now that she has her powers. This would cause major conflicts with both Him and Grim/Mandy. The Ghost Zone may not have solid alliances, or they may choose to break them, so they can sit back and watch the other realms weaken, possibly even going in near the end of the war to gain the most they can for little losses.
Of course Mimi can't just die at anytime, there would need to be a proper build up to it. Regardless, I feel that this would be the most interesting turn of events in any of Bleed's comics, if it ever happens.
Oh, I could write an essay about this xD But, it would fall on deaf ears T.T