What single card do you think will increase in value the most in 10 years

[Great Winfield]

Since this is a baseball forum, I'll lead with a pre-war baseball card that would seem to have catch-up potential: 1939 Play Ball Ted Williams. For some reason, the Splendid Splinter has lagged other inner-circle immortals in recent years (e.g. Ruth, Cobb, Gehrig, Mantle, Mays, Aaron). The Williams rookie has a significantly lower pop (PSA ~1200 plus qualifiers) than the latter three rookies (Mays 51B 2000, Mantle 51B 2260, Aaron 54T 6267).

Looking more generally, top vintage soccer rookies seem relatively undervalued. Yes, Pele (all of his rookies, but especially the 1958 Alifabolaget) and Maradona have skyrocketed in recent years, and Messi and C Ronaldo are still playing and generate interest, but the next tier of greats haven't moved commensurately. Soccer pops are much lower than for other sports, and to state the obvious, it is the biggest sport in the world, so a pickup in demand could have outsize impact. Meanwhile, Messi mania is in full swing in the U.S., and a quick perusal on Ebay suggests that the top watched cards have roughly as many watchers as for other major sports.