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ChadSay, the Billikens from Cuba, the V100's from Canada, a Japanese set (hundreds to choose from), and a Toleteros set from Puerto Rico. Or, if you want to make it exceptionally tough, the Dobles set from Venezuela. Or, you could make it easier and go for a Topps Venezuelan set or maybe the Ovenca set. Actually, now that I think of it, a grand slam like this could be done under a budget relatively easy:
1. Cuba: 45-46 Felices without the Reyes shortprint
2. Japan: My god, there are a lot of sets to choose from. One of the 1948 or 1949 Menko sets would be easy enough.
3. Canada: Any of the 1970's or 1960's O-Pee-Chee sets. Laval and Parkhurst are doable, but tougher, too.
4. Venezuela: Ovenca or one of the Topps sets
Anyway, goals I've set for myself but will never complete:
A card from every country that has ever printed baseball cards.
A complete set from every decade.
A card of every player who ever appeared in the Negro Leagues that has a card. (Man, there are a lot of these guys. I'm pushing a hundred different players already.)
A 1930's type set.
An Oakland Oaks type set.
And, if I ever figure out how to make money, a card of every player that played professionally before 1950 regardless of the league and country they played in.
Of course, this list of goals evolves weekly. 
--Chad