Here is idea for event when invading fallen empire capital all values will be adjusted, this is just a preliminary draft for an idea:

name: legendary hero joined battle
change to fire when: invading fallen empire capital
description: A legendary hero from ancient wars has been unfrozen from his cryosleep by a fallen empire and now he has joined the battle to lead the enemy army
effect: enemy comander is replaced by level 10 paragon

option:
1) text: This dont stop us
description: we continue combat as planed
effect: enemy deal + 50% dmg and morale dmg and get 50% less morale dmg for 90 days

2)text: Try to assasinate him
description: we will try to distract the enemy so that a group of special forces can find and destroy the commander
effect: enemy deal + 50% dmg and morale dmg and get 75% less dmg and 50% less morale dmg for 60 days, event fire after 60 days

if some degree of militarist can choose 3) text: Finally worth enemy, our battle will be legendary, call our best strategist and warriors!
description: we call our best strategist and warriors to defeat enemy
effect: both enemy and us deal +50% dmg and get -50% morale dmg for 180 days

if some degree of xenophobe: 4) text: alien never can be good as our commanders
description: we motivate our soldiers by telling them repeatedly that enemy commanders can never fear as good as ours
effect: enemy deal + 50% dmg but 50% less morale dmg and get 50% less morale dmg for 90 days

name: ENEMY COMANDER KILLED!
change to fire when: 50% choosed option to asasinate comander, 100% if lv 10 comander killed in combat
description: after enemy legendary leader killed enemy forces are in chaos, we use this opportunity to attack
effect: enemy get + 50% dmg and morale dmg for 90 days, if fired due previsious event and commander is still alive enemy commander is killed
1) text: Time to attack!

name: assasination failed
change to fire when: 50% choosed option to asasinate comander
description: we failed assasinating enemy hero and now most of population of (planet name) belive he is choosen to defeat us
effect: 6 armies spawn on enemy side, enemy deal + 75% dmg and morale dmg and get 50% less dmg and 90% less morale dmg for 150 days

all values is I think unbalanced but this is just a preliminary draft for an idea:

name: rebel support
change to fire when: invading planet with low stability
description: due to dissatisfaction with the current government, a large part of the population on planet (planet name) has rebelled and decided to support us in the fighting, now one of these large rebel groups has contacted us and would like weapons from us so that they can more effectively support us in the invasion, however, if the weapons end up in the wrong hands, it can cause us a lot of trouble later with the rebels and crime in the future occupation

options:
1) text: denied
description: we will not put our weapons in the unknown hands of amateurs, it is better to give them to our experienced soldiers
effect: enemy get +5% dmg

2) text: give them old weapons
description: we will not waste modern weapons but we will still support them in their efforts with a few old weapons
cost: 500 energy credit
effect: 90% change enemy get +20% dmg and morale dmg, enemy deal - 10% dmg
10% change on event weapons in wrong hands (event modifier: +25 crime and -10
stability for 10 year on planet name)

3) text: give them the most modern equipment we can afford
description: having a lot of support in the local gurilia militias will definitely pay off for us...
just hope the guns don't end up in the wrong hands
cost: 2000 energy credit, 500 minerals, 100 alloys
effect: 90% change enemy get +50% dmg and morale dmg, enemy deal - 20% dmg
10% change on event "realy bad idea" (event modifier: +85 crime and -20 stability
for 10 years on planet name)

As for event ideas:

1. Species-Species Events (e.g. humanoids vs. reptiloids, cultural clashes, environmental adaptation, biological warfare etc.)

2. Pre-FTL Civilization Events (depending on the technological level of the pre-FTL society: primitive traps, guerrilla warfare, hidden hideouts/bunkers, indoctrination, propaganda war, utter nuclear destruction for atomic age civilizations etc.)

3. Army Type Engagement Events (e.g. biological armies vs. synthetic armies, xenomorphs attacking etc.)

Another would be, depending on the weapon platform some horrific side effects againts the enemy. Such as "making the enemy to red mist" (ballistic or railguns), horrific burns. And of course, give the ability to glass planets if I have plasma weapons.

I would also like some modifiers on planets, like desert planet related events, or jungles in tropical planets and such.

I was playing Reach these days and i though in an idea of some sacrifice in an losing battle like:

If the planet is lost a few pops may escape to some other planet, or an soldier makes an great sacrifice killing off huge soldiers and improving the moral of the battle or empire

I have idea about compatibility modification with mod "Capture Ships" (2826506007). Need form event tree with capturing broken ships and using for this army instead of Constructor or separate capturing on two phases with capturing and repairing ship debris. We must have positive and negative variant of combat realized by chain of events with some random reward (like capturing arsenal of the ship and get some amount of armarmnment) with capturing ship in positive finish and loosing selected army and constructor ship in negative ending of event chain.