Charley and Science would be a messy argumentative showmance worth treasuring
I would've loved Grace to be in BB6. Her vs Saskia vs Makosi would've been even more heated than the original. Lea was nearly in BB6 too so it would've been incredible to see her emotionally sink even lower. And with the massive personalities in BB6, she'd have such a difficult time in trying to feed off the sympathy of others since all of the cast were pretty selfish in some way. The only downside is the possibility of her walking early.
I also would've liked pretty much any awful disgusting people from BB/CBB in the same series as Pete Burns since he could rip anyone to shreds. On the subject of CBB4 I also wouldn't have minded Chantelle in BB6 since she'd make for a much better regular for that series than Anthony who should've just stayed as a background to Makosi and Craig's stories.