The ability to pull data from scoreboard objectives would allow for a lot of unique interactions when it comes to mob spawns.
For example:

By creating a scoreboard objective minecraft.mined:minecraft.stone one can keep track of how many stone blocks a player has mined.

For example in breakevents.json

breakevents.json
[
{
"scoreboard":{
"objective": "gt(minecraft.mined:minecraft.stone,10)"
},
"block": "minecraft:stone",
"result": "deny"
}
]

would deny mining stone blocks if the player has already mined 10 stone.

Or in spawn.json,
spawn.json
[
{
"mob":"minecraft:skeleton",
"scoreboard":{
"objective": "lt(minecraft.picked_up:minecraft.diamond,5)"
},
"result": "deny"
},
{
"mob":"minecraft:enderman",
"scoreboard":{
"objective": "lt(minecraft.picked_up:minecraft.diamond,26)"
},
"result": "deny"
}

]
gradually increase allowed mob types based on how many diamonds a player has picked up.

Add in the ability to create your own scoreboard objectives and command blocks, you can now create dynamic events/phases.