I do think Minecraft needs an extra level of difficulty. Better AI? Rarer-tougher mobs? Balance mid-end game? I don't know. Meanwhile I use commands blocks on my singleplayer world to make mobs a bit stronger according to distance and proximitty of other mobs:

[These are my notes on that:
Mob support ===========

Zombie gains
- Resistance I     by Enderman
- Speed II         by Witch
- Jump boost II    by Creeper
- Strength I       by 2000-4000
- Strength II      by 4000-6000
- Regeneration I   by 4000-6000
- Strength III     by 6000+
- Regeneration II  by 6000+

Spider gains
- Speed II         by Enderman
- Strength I       by Witch
- Resistance I     by 2000-4000
- Resistance II    by 4000-6000
- Fire Resistance  by 4000-6000
- Resistance III   by 6000+
- Invisibility     by 6000+

Skeleton gains
- Invisibility     by Enderman
- Regeneration I   by Witch
- Resistance II    by Creeper
- Speed I          by 2000-4000
- Speed II         by 4000-6000
- Speed III        by 6000+

Creeper gains
- Regeneration III by Enderman
- Speed III        by Witch
- Resistance I     by 2000-4000
- Resistance II    by 4000-6000
- Resistance III   by 6000+
- Jump Boost III   by 6000+

Witch gains
- Resistance II    by Enderman 

execute @e[type=Enderman] ~ ~ ~ effect @e[r=15,type=Spider] 1 60 1
execute @e[type=Enderman] ~ ~ ~ effect @e[r=15,type=Zombie] 11 30 0
execute @e[type=Enderman] ~ ~ ~ effect @e[r=15,type=Skeleton] 14 10 0
execute @e[type=Enderman] ~ ~ ~ effect @e[r=15,type=Creeper] 10 30 2
execute @e[type=Enderman] ~ ~ ~ effect @e[r=15,type=Witch] 11 60 1

execute @e[type=Witch] ~ ~ ~ effect @e[r=5,type=Zombie] 1 60 1
execute @e[type=Witch] ~ ~ ~ effect @e[r=5,type=Spider] 5 30 0
execute @e[type=Witch] ~ ~ ~ effect @e[r=5,type=Skeleton] 10 60 0
execute @e[type=Witch] ~ ~ ~ effect @e[r=5,type=Creeper] 1 10 2

execute @e[type=Creeper] ~ ~ ~ effect @e[r=5,type=Zombie] 8 20 1
execute @e[type=Creeper] ~ ~ ~ effect @e[r=5,type=Skeleton] 11 60 1

testfor @r[rm=2000,r=4000]
effect @e[type=Zombie] 5 10 0
effect @e[type=Spider] 11 10 0
effect @e[type=Skeleton] 1 10 0
effect @e[type=Creeper] 11 10 0

testfor @r[rm=4000,r=6000]
effect @e[type=Zombie] 5 10 1
effect @e[type=Zombie] 10 10 0
effect @e[type=Spider] 11 10 1
effect @e[type=Spider] 12 10 0
effect @e[type=Skeleton] 1 10 1
effect @e[type=Creeper] 11 10 1

testfor @r[rm=6000]
effect @e[type=Zombie] 5 10 2
effect @e[type=Zombie] 10 10 1
effect @e[type=Spider] 11 10 2
effect @e[type=Spider] 14 10 0
effect @e[type=Skeleton] 1 10 2
effect @e[type=Creeper] 11 10 2
effect @e[type=Creeper] 8 10 2

testfor @r[rm=3000]
entitydata @e[type=Creeper,tag=!a] {powered:1,Tags:["a"]}
entitydata @e[type=Witch,tag=!a] {NoGravity:1,Tags:["a"]}

](#spoiler)