What would I suggest? Well, I think that Collective happiness type from Adamant and AIC mods could be a good alternative to current Neutrals. This type is quite indifferent to outer events, and doesn't care about treaties at all, which conforms to my understanding of "neutral". However, it doesn't mean that it will be the best choice.
Code:
Max Positive Anger Change := 50
Max Negative Anger Change := -50
Homeworld Lost := 50
Any Planet Lost := 5
Any Planet Colonized := -10
Any Our Planet Captured := 5
Any Enemy Planet Captured := -20
Any Ship Lost := 0
Any Ship Constructed := -1
New Treaty War := 0
New Treaty Non Intercourse := 0
New Treaty None := 0
New Treaty Non Aggression := 0
New Treaty Subjugated (Sub) := 0
New Treaty Protectorate (Sub) := 0
New Treaty Subjugated (Dom) := 0
New Treaty Protectorate (Dom) := 0
New Treaty Trade := 0
New Treaty Trade and Research := 0
New Treaty Military Alliance := 0
New Treaty Partnership := 0
Battle in System - Win := -5
Battle in System - Loss := 2
Battle in System - Stalemate := 0
Battle in Sector - Win := -10
Battle in Sector - Loss := 4
Battle in Sector - Stalemate := 0
Enemy Ship in System := 0
Enemy Ship in Sector := 0
Our Ship in Sector := -2
Our Ship in System := -1
Enemy Troops on Planet := 1
Our Troops on Planet := -1
1M Population Killed := 1
Ship Lost in System := 0
Ship Constructed := -1
Facility Constructed := -1
Planet Plagued := 50
Natural Decrease := -30
Natural Decrease for Other Races := -10
edit: also I think it's worth to tweak some numbers here: "Battle in System -" parameters, "Any Enemy Planet Captured" and "Natural Decrease" items could be lowered slightly.
edit2: indeed I expressed myself with ambiguity
To clarify: Neutrals shouldn't be emotionless because this trait cost some RP, and current Neutral type allow you to be always jubiliant and don't think about glassed planets, etc (as douglas said). I think it's too cheesy.