portal
A Minecraft launcher where you pick a server, not an instance — clicking one provisions whatever it needs and drops you straight in.
Offline launch works end to end. Signing in with a Microsoft account was abandoned; the launcher runs offline profiles.
Inverting the model
Every other launcher makes you know in advance which modpack a server wants. Create the instance, find the pack, install it, launch, and only then connect. The thing you actually wanted — that server — is the last step.
Here the server list is the home screen. Clicking a server provisions the pack it needs and launches into it, with no main menu in between.
Why it can work without modifying the game
The stock client accepts a flag that joins a server directly on launch. That single flag is what makes click-a-server-land-in-the-world possible with an unmodified client.
Mods still cannot be loaded into an already-running client — the JVM will not have it — so the effort goes into making the provision-then-launch path fast enough that the distinction stops mattering in practice.
Servers that describe themselves
Nothing in the protocol carries modpack information. But the server status response is JSON, and the vanilla client ignores keys it does not recognise, so a server can simply add one describing its pack — and every existing client keeps working exactly as before.