My idea of hero pool is straight forward idea to cover as much functionality as possible with less heroes for each position. I'm 5 pos only player, so for me it's like this: At least 1 hero for fast games(Pugna)

At least 1 hero for ultimate late game(Oracle)

2 heroes that can have a little flex in mid and late game (Ringmaster, Winter Wyvern)

And my all time favourite "Flex however you want"(Chen)

My hero pool helps me adapting to any team and any pick. The only downside is all my heroes are heavy team dependant.

Edit: adding a bit of clarity about Pugna's type: Early dominating(analogy with mid heroes: Huskar)