General Boss Ideas

1. Grab: The boss would be able to grab and toss certain things, possibly grabbing medikits and making them ways to damage those that are subject to the wrath of it exploding.

2. Player attacks: The boss would be able to use player abilities to attack.  Some changes may have to be made for the ones that can't miss.

3. Lower tiered boss revenge: Old bosses given new abilities, like the battlecruiser being made faster with the redline reactor for faster speed for a time, hurricane missiles, warps around, and has a yamato cannon to create rough ground.  This is an example, of course.

4. Multi Parted bosses: Bosses that have other parts attached to them.

5. Boss rush: this may tie in with 3.

6. One full out example using the ideas above
A modified overseer made invulnerable with power orbs designed of the first tier bosses, and they are gonna try to take care of you by eventually making copies of your guys.  Of course, you don't want to be killed, you don't want any inferior copied, and that guy may be working with Amon, I dunno.  So, in order to get there, you have to first destroy a ship that is a pirate's ship.  It's here to first get your feet wet with the orb abilities.  There will be orbs of your character it will have circling it, protecting it from harm.  These have half the hp of a BC, and there is no need to scale the hp with the amount of players, if there is going to be orbs based on the number of players.

Anyways, these orbs will not go away permanently, but while they are up, the boss will gain from the orbs, and they have skills equal to your own.  The orbs may also gain from the abilities, and may have a weak attack based on your weapon type.  Once the orbs are gone for the time being, the boss will use abilities that are like the tier one bosses, but modified.  Most notably is the laser sweep, which sweeps now across the station, and the void ascension, which warps out and in dropping the void clones that will have stronger weapons and have a skill.  It will also have minions from the previous bosses.

So the boss is defeated and you head to the overseer, which is made invulnerable by the "Boss orbs".  The boss starts off with creating changelings that turn into players.  these changelings are all around slower and weaker than your characters, but they are fairly beefier.  It also fires contamination at an area, which prevents anyone who happens to walk into it from using medkits for some time.  The boss orbs can give it attacks for some time.  As the orbs are defeated, the invulnerability shield weakens.. but the overseer gains boss abilities.  Once the shield has fallen, the overlord will be avalible to attack.  It will occasionally drain the life of its own minions.  

Once it gets low HP, zealots start rushing in while the boss turns invulnerable.  it will start extracting zealot essence or what have you, as they are faced atthe overseer while they turn red or something.  they become harder to kill.  you stould focus on killing as many as you can, though, as every alive zealot increases the effectiveness of the Shadow Lord, and his reformed body.  

This is a hybrid behemoth, and it uses the shadow lord's attacks, but he now has a few new tricks and modifications.  For example, his shadow orbs spawn minions upon hitting something(or exploding), the voids that spawn out are tougher and spawn out more types, in addition to providing him one more shadow orb every time he calls for shadow orbs, and the firing patterns are different as well.

Straight shot: waves as it moves.
Spread shot: fires off in a direction like a straight show with all of them in one shot, before having all of them spread out in 360 degrees.
Scatter shot: happens as normal with the 360 degrees and all, until they go off the platform, where they turn at the nearest player for a little bit while still moving forward.

He also has his "void ascend" attack, although this time they can use their standard hero ability, abd they have something better than just a marine gun.  They also have stats like a level 15 with 9 points in life and 6 points in armor.

However, He has Psi orb, which will damage those who get close to it as it moves across, Constricting slime, which will slow players and damage them slightly over time, sacrifice, which will lower a minion's hp to one to fuel his energy requiring attacks, medkit overload, where he targets someone and makes them lose all of their medkits and take 1 damage for every medkit that they have, and void armor, where everyone that is not aligned with the heroes gains +200 shields.  He also gains some like and every for every unit that dies, and more if it is a hero.  To make up for this, the buffs he gets per hero are less.

His energy attacks add in a whole new level of trouble.  First of which is shadow cannon, which sacrifices damage and accuracy(Shots deal 10 damage and don't track the target) for sheer number of shots(5 energy per shot, so 20 energy would equal 200 damage on a moving target).  These shots are countered by being agile enough or, like the original version, having good armor. Zerglings should be a top priority, as the shadow lord will grab them when it moves around. He'll occasionally toss all his zergling at the fastest player, trying to trap them in it. They'll deal minimal damage, but they will follow said player until they hit.

Anyways, when it goes down, you've almost gotten the boss down, now you just need to do the final blow: The TITAN. The TITAN looks like this:
https://gyazo.com/5358dda7972fd35e0a617832cd575b6b
Those diamondbacks, by the way, are kinda a reference point.
Here is a larger reference point.
https://gyazo.com/352ee192c2793935a3e5d83ae20d8e58

Anyways, It starts off like that, but you are actually climbing it in order to get to the elevator, which has been jammed, and it's on the other side of the boss. It is a rather large elevator, presumably used for carying heavy objects up, when all of a sudden you hear a rumble. https://gyazo.com/b41d206dbecbbdaba5ddfeb2280ac54a
This is where you would be when the rumble happens. Then the boss's eye lights up, and it turns to face the other direction, eventually like this, with your guys still on the metal floor in front of the eye.
https://gyazo.com/a1801951451a263b7ba3bed26723fee1
Eventually, it slams the ground, shaking you off and onto the platform.
https://gyazo.com/cad36aea6758e521cb8db993311ed53a
The attacks are will be continued later.

My proposals as boss:
-Alarak
-Ma'lash
-Hybrid Nemesis
-Void Thrasher

For Raids:
-Amon, obvious!!! With his stones.