>>986718
How do you see it playing out? Sell us on it OP.
Personally I think the biggest problem it presents is that Fred and Velma are the two most obviously competent and intelligent members of the gang, while Daphne and Shaggy are dimwitted monster-bait. With a traditional Fred/Daphne and Velma/Shaggy pairing the scenes are easy to write because you have one intelligent character to give exposition to the audience and one dim character there to crack jokes and do comedic freak-outs when the monster shows up.
Pairing Velma and Fred would turn the gang into one hyper-competent (but boring) mystery-solving power couple and one pair of amusing but ineffectual cowards.