I think it would be interesting to have boss(es) that have random(from a pool) skill and/or adaptive skill(depend on what race or class attacks it)
Now, I'm not gonna design too much specifics because balance would be a pain, but here are some examples of adaptive skill(random would be simple conceptually because it's just random, just have to design an interesting pool)

Edit: It will come with two passive abilities, IMI and its version of Cerulean Immobilizer(credit to DarkMar for pointing out this weakness), let's say set it to 125% lock damage(whatever that might be) change to 150% when Lazuli triggers. Mylari will "remove" damage from players to avoid problems. changes to 175% when both lazuli and Uldinan are triggered.
Racial Adaptive: This NPC gains an ability the first time a member of each race attacks it:
Aerlen: Gains +50% defense
Human: Rank decrease by 10%(thus damage cap)
Inergon: Chance on hit: drains attacker of 100 energy.
Konqul: Gains +50% attack
Sillix: Gains +5% damage cap(+10% if human is triggered) and +40% atk and def
Vygoid: For each player that deals 50% of locking damage to this unit, gain 10% attack and defense.(note 100% is still just one stack, so if it's a 12 man, it will be 11 at most. 50% instead of actual locking amount because otherwise everyone would deal 1 shot short of lock and this would be useless)
Drannik: Chance on hit: reduce attacker's NPC attack by 50% (stack-able, removable). 1 hour
Genetarr: Chance on hit: reduce attacker's attack and defense by 10%, stackable, non removable. 30 min
Kronyn: Modify Vygoid to each 30% locking damage, stacks. (so 30% is 1 stack, 60% is 2 stack and 90% is 3 stack. 89+89 is still just 2+2=4 stack)
Lazuli: Increase max and current hull by 50% of its max
Litheor: Chance on hit: spawns a Deep-Phase drone
Mylari: Chance on hit: repair 10% hull, cooldown 1 hour
Taltherian: Each time this is hit, gain 50 attack and 50 defense
Uldrinan: increase all other ability's effectiveness(knoqul/aerlen become +75%, human become -15%, inergon become 150, sillix is 5/45 and so on, basically 50% boost compared to original, increase chance on some chance to hit effects)
Xecti: Chance on hit: gains 20% defense for 30 minute
Zolazin: players with less than x scan cannot attack this unit (x can be whatever number, depend on rank of boss)

Deep-Phase drone: If you attack the boss while this is on your BT: your race is considered as a random race when you attack(meaning it might spawn a new ability. Thou you might also randomly roll a race that already exist)

Note that the ability is gained on the first time it is attacked by a player of a certain race and stays permanently and affect all subsequent attackers. One interesting aspect of an adaptive boss is that the order in which members lock onto the target would affect the difficulty of the boss as some effect would be worse than others(for example, probably nobody would really care for the konqul one but human/inergon would be a pain always while talth would be bad if it is applied early) Ofc, such a boss would need to come with IMI. Doesn't really need any other ability, it will just grow nicely.
I apologize for attaching Kronyn to Vygoid, but they are both research races, they just belong together XD.
All numbers are only meant as examples, exact effect can differ, this is just the concept. Kinda bit more than I can chew when I started down the race list, it's very colorful.
There could also be a profession based effect, and a boss could have both.

-1, bad idear

for me this just says, let 1 player do 95% of the dammage to it, so it doesnt gain multible abilityes
then share it, and have 3, 4, 9 or 11 players tap it before taking it down

if you want team work, it should instead be
have this ability..... (from your list) until it have been hit by a player of that race..

You know there is an NPC called Roathir Cerulean Base? It have a helpful ability:
Cerulean Immobilizer (Will sometimes fire its Cerulean Immobiliser, preventing the attacker from firing again, Will only be used on attackers who have done more than 130k damage to the base. Lasts 30 mins and cannot be removed by a Containment Cage.)
So this NPC just needs a similar effect in addition to the IMI as default.
Good point thou, forgot about that "exploit", now it's patched, anything else?

I know them all to well, think I hunted down 10 to 15 Roathir Cerulean Bases
last time we had that legion mission

putting the Immobilizer on the Boss, so atleast 6+ players would have to hit it helps, but doesnt realy stop the exploid

we have 20+ Taltherian, and 5+ Uldrinanin legion atm, so for us it wouldnt realy be hard to take a NPC like this one down without activading multiple abilityes

Probably should be a 12 man so it takes 8 to down most of its hull/shield(assuming no Lazuli trigger, Lazuli trigger will require at least 11 depending on when immobilizer triggers) It's true that common class will likely take a large chunk of it before it get too complicated but when the others want the reward it might get more fun. Who are allowed to hit which one would become part of the team work. Thou considering it might not get too many effects with proper team work, maybe the power of each effect can be bigger so that it become a reasonable challenge with say 4 activation. Sadly most talth and uld are of same profession so adaptive profession ability won't do much more neither.
Alternatively, I guess it could come with some painful original skill that gets weaker with each racial and/or profession skill spawn. Maybe it should gain a 1000% defense buff every 20% down if there is insufficient number of racial on it or something.
Seems like a lot of details needs to be worked out, all I thought was that some kind of changing boss would be fun and then all this XD

I do like the idear, about a boss with adaptive skills Uy23e

but main problem is, as players gain rank, they tendt to drift towards 5 races

Aerlen - for base defence
Konqul - for the attack bonus
Taltherian - for planet boosts
Litheor - for deep probes and scanning
Uldrinan - for the special structures (Uldri Power Node (Size:1, Research:1, Artifact:+5%)) and Uldrinan Onxicant

so in most high rank legions you wont find all races, unless some one is working on a medal or legion mission


that's why I thing reversing things would work better
if you dont have a Lazuli in legion, to hit it - you would be stuck with a +50% hull boost on the NPC, etc etc etc

= you would need 16 difrent races in legion, to eliminite all it abilityes




if you want to keep it adaptive
the alternative could be, that once a race have done 10%+ dammage to the NPC, it starts to be protected from that race

the easy way would be a dammage reduction around 3.3 * dammage % done by the race = 99% dammage reduction vs that race once it have done 30% dammage

that way, you would need atleast 4 difrent races to hit this one, to be able to kill it