My definition of what is (or isn't) a "big bang" theme is simple - is there a cartoon series to accompany it? If there is, then it's a "big bang" theme; if there isn't, it isn't. Simple :-)
As for the next non-ip/non-"big bang" theme, I'd love to see a return to Classic Space, particularly something with a back story around exploration and adventure. Unfortunately, if we do get a new Space theme, it's not going to happen quite like that. Lego's current philosophy of encouraging conflict-based roleplay wouldn't allow it; they feel the need to always provide "good guys" & "bad guys", in conflict with each other. But, as long as there are spaceships involved, I'll be happy.
I've been buying from Bricklink for a long time (since the 'brickbay' days); I tend to buy from UK sellers if possible, then European, then US/Canadian/RotW in priority order. This is based on my experiences - I've had fewer mail-related issues with the more locally sourced parts. In many dozens of purchases, I don't think I've ever had one that hasn't worked out fine eventually - any problems I've had (missing parts, sub-standard quality, etc.) have been easily resolved by emailing the seller. The only frustrations I've had have been with very long delays in getting the stuff I've ordered - sometimes because the seller can take a week or two to get the items in the post, and sometimes because of very slow postal services (and, in a couple of instances, the packages getting held up in foreign postal workers strikes).
So, in summary - Bricklink itself is great; some Bricklink sellers are better than others, but all (in my experience) come good in the end; postal systems are variable & not to be relied upon if you're in a hurry for parts. Minimise the distance the parts have to travel if you're in a hurry for them. Build up a relationship with a couple of sellers & use them for most of your purchases. The sellers I've grown to use most are people whom I've met in real life, so that helps enormously.