For a nostalgic reunion-adventure like that, I'd think it's a perfect excuse to run them through the original grandaddy of Ravenloft adventures, I6 itself (or the 2E HoS equivalent).
Not with any of the original Pick-4 plotlines, mind you -- not at
those character levels; they'd be meat-cakes in two rounds if they actually
did fight Strahd

-- but something along the lines of a hit-and-run rescue mission, undertaken during daylight, might be ideal. The challenge would be for the PCs to get past the traps and guardians of the upper castle, recover a victim they've been sent to retrieve, and make it out alive again ... with enough time left
before sunset to put serious miles between themselves and an irate vampire-darklord.

"Who [u]cares[/u] what the Dark Powers are? They're [i]bastards![/i] That's all I need to know of them." -- Crow