Pics in press today of David filling up petrol cans, so looks like it's definitely going to be him that causes (or starts) the disaster. I am very keen indeed to know exactly where this will kick off. It'll be surely being filmed in the next week or so which presumably means the stunt will happen in the first few weeks of November.
I'd advocate a relatively high body count (though we know only Michael will likely die). If it were me I would kill off:
Michael (die a hero, Gail devastated and later to discover her son was responsible)
Billy (actually murdered/left to die by Vinnie/Phelan after discovering the scheme)
Sonia (not before affair reveal, tearing Nazirs apart)
Freddie (paving way for Audrey/Ken romance)
Sinead (paving way for major grief plot for Ches)
Izzy (paving way for single-Dad in Gary and relationship with Sarah)
I'm tempted to say one of Luke, Steph or Andy as well, because they've never really gone anywhere and their deaths could be catalysts for other character stories, but I would probably chance giving them a meatier plot later and see how they go.
At least that way it'll make doing yet another explosion actually mean something and spark some storylines. Clear some of the deadwood out. Sadly though I don't think we've ever seen an event kill more than 2 main characters at a time on CS, maybe they don't want to upset the audience too much.