As more expansion sets get added, it will be interesting to see if you can trade cards between sets and across expansions, provided they meet the requirements
First Appearance of Trainer Cards The Base Set introduced essential Trainer cards like Professor Oak, Bill, and Energy Removal
Id recommend leaving the Monoliths behind and instead run a lot of cards that nobody plays because the activation cost is too expensive
Skullclamp The story goes that Skullclamp originally gave +1/+1, but was tuned down to +1/-1 to make it more fair right before print
Iconic Card Showcase Giant Spider (Keywords: Reach): The original, classic Reach creature
Moving on to BLUE, we've got the Kang Dynasty and a Mythic Immortus, Master of Eternity, depicting one of the most powerful of the many Kang's variants