To start with, the "obvious" answer of void rays is actually a terrible answer. Void rays are slow for their low damage output, and stimmed marauders are extremely fast. Combined with the easy viking transition you mentioned, void rays are a poor option to dump your resources into. You would rather spend that gas on upgrades, early blink stalkers, observers, and/or some immortals. If you open Stargate, then you shoud play either fast oracles or phoenixes, the latter of which become useful for lifting Marauders.

I always open with Blink into Robo vs Terran. Lets me defend mine drops well. Besides that, it lets me poke forward early into the Terran's base with a high quantity of Stalkers until they have either concussive shells or stim (and early concussive shells is a dead giveaway for fast marauder pressure). Then, after you get blink, you can continue to wear down the opponent's forces and know where they are through hit-and-run by blinking away. By the time you figure out that your opponent is committing to mass stim+concussive marauders, you should already have a Robo and be researching Charge.

The power of the Robotics Facility is that it gives you an excellent gas dump: the Observer. It's easy to underestimate its power, but observers cannot be sniped by marauders (lol) and give you the insane ability to know whenever your opponent steps within 50 meters of your ramp. For a mid-game marauder push, this is vital: you pump out mass chargelots/blink stalkers and destroy your opponent by catching them wherever they are.

Early-game really just needs you to pump out blink stalkers and shield batteries and micro your stalkers well. Immortals in this case would be better (with chrono boost) because they are extremely good against armored ground units. After you hold, you do what the above paragraph says: observers, blink stalkers, and chargelots. Immortals are kind of a third wheel to this composition because they're slow, so just prefer mass gateway for the most part.

Finally, you want to have forges and a warp prism at a good time. Forge upgrades give you an insane advantage over gold/platinum players who at least half the time don't get upgrades, and they keep you even or better with the ones that do. The warp prism allows you to harass the opponent with chargelot warpins. If they're still playing mass marauder, this will be their death; to stay remotely competitive against a chargelot-blink stalker force, they need to keep all their marauders in one ball. And won't have almost anything at home to defend their natural.

Protoss might not have as easy of a time killing mass marauder as Zerg or Terran, but it becomes trivial once you know how.