Was writing this for another thread, and then decided "Well that's a bit off-topic. better post in in the NG section"
This is basically my thoughts on each game's strengths and weaknesses, and gives a basic idea of what I'd like to see in NG4.
Ninja Gaiden: Black
+ World immersion & exploration
+ Very calculated, technical combat. Nothing left to randomness. Your skill alone dictates success or failure
+ Hidden secrets (Unlabored Flawlessness)
+ Varied enemy designs and strategies
+ Excellent boss fights
+ Great soundtrack
+ Good weapons
+ Overall, good inventory system
+ Great replay value with costume unlocks, mission modes, hidden content, new and exciting difficulty levels
- Selective purpose weapons (not every weapon could be used in every situation)
- Lack of perpetuity (weapons/items/scrolls/ninpo all reset, no level select, etc.)
- Not as full as it could be (alt. characters, level select, co-op etc. would be welcome in some form)
Ninja Gaiden Sigma
+ Dual katanas
- Every other change made from Black
Ninja Gaiden 2
+ Grand and beautiful environments
+ Visceral, sharp and fluid combat
+ Brutal gore and a sense of carnage
+ Amazing weapon selection. All feel great and can be used in nearly any situation. Lots of freedom
+ Great soundtrack
+ Great mission mode with DLC
- Combat can be random, unbalanced and cheap (enemy hitstun is too random, behaviors unpredictable, bad spawns, projectile spam, etc.)
- Bad camera
- Lack of replay value (no new enemies on higher difficulties, bad costume unlocks, etc.)
- Lack of perpetuity (same as in Black)
- Horrible boss fights (seriously, horrible)
- Not as full as it could be (alt. characters, level select, co-op etc. would be welcome in some form)
Ninja Gaiden 2 Sigma
+ Better costumes, weapons and character selection than NG2 vanilla
+ Level select
+ Co-op
+ Most good stuff from NG2 vanilla still here
- Purple mist/lack of gore
- Weird spawns and some new, ludicrous enemies/AI patterns
- Shitty mission mode
- Most bad stuff from NG2 vanilla still here
Ninja Gaiden 3
+ "A Masked Curse" (the Regent's theme) is a pretty cool song
- Everything else
Ninja Gaiden 3: Razor's Edge
+ Good gore
+ Decent weapon select
+ Good costume/character selection
+ Level select
+ Co-op mode
+ Decent mission mode
+ Sense of perpetuity (character unlocks/upgrades retained for chapter challenge and the such)
- Combat is at an all-time low, excluding NG3 vanilla (far too random and bullshitty)
- Bad inventory system
- Weapons are gimmicky in use and best strategies revolve and mindless spam and luck
- Boring environments
- Lame, uninspired, annoying and cheap enemies
- Lackluster soundtrack
- Horrible boss fights