The core can open up and still not have us get off cradle. There have been many suggestions for that. A portal, wouldn't work too well.

However you do forget the fact of how the clockworks do work. Lore-wise, it takes in little bits and pieces from other worlds and pieces them together within its inner workings. So, say, we aren't exactly fighting the Morai on Isora, however the clockworks could take a small piece of our world (Morai and all) and construct its own bit. Bam, now we have a lore-friendly way to fight on Isora against the Morai without ending a story and keeping with the current lore.

In games such as Skyrim or other open-ended MMOs, it's fine to have stories or "quests" that have a beginning and an end. The route that Spiral Knights originally had was very much open, only having the Arcade as playable content. Now-a-days, however, we have a linear mission system that, once it ends, it ends. Leaving Cradle ends the "gameplay" of spiral knights, which is why I am against it. It also ends the game "lore-wise", but that's less important. We need to be on Cradle because it has the clockworks - which is what the game is about. Leaving Cradle is leaving the clockworks, which thus makes the game less of what it is supposed to be.

Okay, here we go:
Cradle is not a planet - it is a piece of mechanical genius. The reason it all works is because there is something very powerful at its core, something that us Spiral Knights could detect from very far away and sought out to find so we could win our war.

There are not more "cradles" out there. You can have other clockwork factories around, however we will not have the same clockworks with the same enemies or rotations. It is true we could pull a Halo and after escaping cradle discover there's like, 11 other clockwork spheres floating around in space, but without a powerful core why do we need to land on them, especially if we harvest this core's energy?

The story behind Spiral Knights is summarized as such: We're losing a war, we get a very high energy reading, we seek it out and find it on Cradle. Some unknown force destroys our warship, and we abandon ship, using escape pods to land on the "planet" surface.
The only reason we would leave Cradle is if we had a spaceship back and that energy source. But us Spiral Knights haven't been able to do much except scavenge one reactor from the mess of a ship and stare at the core terminal.

Leaving Cradle ends the gameplay that is set up in Spiral Knights. No longer are we going down to reach the core to rescue alpha squad. No longer are we trying to open the core to fix our ship and win our war. No longer are we fighting off an army of natives that seek to destroy us at all costs. Leaving Cradle isn't the end of the Spiral Order, it is an end to the game.

However, we can tie in other planets to Cradle because of how the Clockworks works. We don't go to other worlds, other worlds come to us.

A lore-friendly, gameplay friendly, and future-story friendly way to add content such as this is to add it to random clockwork rotations. The not-so-good way would be to physically leave Cradle.

Leaving the clockworks is leaving what built this game. We can have random levels with random enemies of random statuses and it is all built into the base lore of the game. We have mechanics working on the planet to maintain it too, so that's why it's lasted as long as it has.

The same gameplay could take place elsewhere, but it just won't be spiral knights anymore... It'll be more of a runescape-esk thing with Spiral Knight characters.

Bottom line is that, gameplay-wise and lore-wise, leaving Cradle will be harmful. No longer is the main plot-point in effect - the whole stranded thing. No longer are we hacking and slashing to the core because we're not on a clockwork planet anymore. The game becomes a much more generic hack n' slash opposed to what has made Spiral Knights great.

Also a design that Three Rings made early on was not to separate players at all. Doing what is asked would be separating players who are not playing the game at the moment. It's why we all default to Haven instead of different subtowns.

Design-wise, it's not a good idea. Maybe a different game would provide a better situation, but Spiral Knights in its current state is by no-means a game where we can leave Cradle.

The Clockworks is built as a prison, initially to protect what is in the core. We would need to write a completely different story as to why we are on another clockwork planet, and why another clockwork planet exists. What I'm getting at is that in the time it would take to make a whole new lore/gameplay area, Three Rings could make a completely new game.

Leaving Cradle leaves behind our Clockwork Planet, the beasts, the slimes, the gremlins, the constructs, the undead... And that means half our family bonuses don't work anymore. Yes, we can have beasts and slimes and maybe undead on another planet, but gremlins are native to Cradle, and constructs are their creation.

Community-wise, we're split. Not good for game design. Gameplay-wise, we have to create a new system unless we want to be super boring and just clone cradle and call it "cradle 2: electric boogaloo" and just make things "harder". Lore-wise we would not come back to Cradle after leaving, and in MMOs there cannot be a real "timeline" - everything has to be able to happen several times. The Clockworks can generate the worlds it takes from many times over and over. Gremlins can rebuild the roarmulus twins over and over. We cannot, however, win the same war not on Cradle over and over. We cannot leave Cradle and still have recruits landing in from the crash. It's the same reason you can't go back to rescue camp as a Vanguard, the rescue camp isn't generated by the Clockworks.

I believe we can have harder content. I believe we can open the core and not end Spiral Knights. However, I do not believe we can leave Cradle and still be playing Spiral Knights. It's the same reason we can't leave Skyrim in the Elder Scrolls V, because no longer are we in our game.

You're looking at this as "more content!" rather than "is this healthy to every aspect of Spiral Knights?" More content is healthy to Spiral Knights, but separating the community and pulling a Halo, plus redesigning an entirely new planet with its own personal lore is not healthy to Spiral Knights.

Hence, I stand by the idea that we can pull the Morai into Cradle through means of how Cradle works, but we are not actually on Isora for the same reason we aren't actually in Almire.