Well, if you have been following the Katastrophe event and checking the forums, you will already know that it comes with a secret boss battle that you can activate somewhere. I got to say I was surprised about it, and made me wish there were more like this, so:

This suggestion simply is about making this for different places ingame and creating "secret" content that we could trigger in specific places and/or missions.

Candidates for possible secret bosses based on popular requests:

> Vog / Vog Cub: May be able to be triggered in FSC, stage 4 after the first forced fight (where Wolvers spawn) or elsewhere else...? Maybe Vog or a Cub of his/her could have been hidden while Vanaduke's castle was included into the Clockworks, and is dormant there... An alternative place may be for example in The Gauntlet mission, or in Legion of Almire. I suppose that, as the Black Kat boss spawn those seemingly random Curse bullets, Vog (Cub) may be able to also summon similar ones, but inflicting Fire. Maybe the boss may even be against the altar to Vog where Vanaduke got cursed, and Cubs would spawn too, making a similar fight to Maulos?

> The old big Jelly: That hidden Jelly that was around in ancient times. Looking at the Black Kat boss, I wonder why such a thing is still not again with us. It isn't needed that it is the same - in fact it would be better if it was harder most likely! Summon place may be, of course, The Jelly Farm stage. The key item needed may erase the wall blocking access to the old battle place, and start with the push of a button.

> Tortodrone: One of the most wanted comebacks could be a really hard boss. The Black Kat boss isn't something that can be done alone easily, I imagine all these fights would be team oriented, and Tortodrone I think it had an AI that basically needed two people due to better tracking than a Trojan. IMFactory, a possible trigger place spawning randomly in Decon Zones, or "factory" missions could be good places to choose where to hide this possible Secret Boss.

> Alpha Squad Member (Illusion?): Even though I have seen it suggested and considered as the others, this one may be controversial - I think they would be able to spawn in The Sanctuary. After each of the four Shadow Lairs, along with the Echo Stone, there may be another one where you could "reach" to each of the members of Alpha Squad, and request a "training" battle with each of them. They would be 4 separated bosses, each with their own strategy, and you would fight them in a separate place.

About the key items to trigger them, I would be a bit lost:
> Vog / Vog Cub: Maybe a rare drop from Vanaduke? Something like "Cursed ashes". There are games where beating a "hard" boss would give keys to "raid" bosses. Also, if fight were in FSC, maybe you could fight only Vanaduke or only Vog (Cub)
> The old big Jelly: Maybe a rare drop from Giant Lichen Colonies or a sort of common drop from the Toxilargo. Something like "Acidic core chunk".
> The Tortodrone: Maybe a really rare drop from high ranking Gremlins - C42 minibosses, mortars, recons, Seerus. Something like... "Prototype storage key"?
> Alpha Squad Member (Illusion?): A really rare drop from Swarm mobs. Maybe only avaiable to get from Swarm mobs spawned outside of forced enclosed fights. Something like... "Gleam of hope", homage to Pandora's Box, where after all evil in the box was unleashed, a small like called "hope" was left.

And about what you could get... all possible prices are left to OOO's consideration. But really hard secret bosses tend to have nice prizes...

I am bad at ending suggestions.