These three types have been done to death. How about some new sensations?
I have a few ideas:
1. Vibrating switches. They vibrate slightly when pressed.
2. Buzzing switches. They emit a buzzing sound when pressed. This can be combined with vibrating.
3. Zapping switches. They produce a small electric shock to the fingertips when pressed. This can be combined with vibrating and/or buzzing. Don't use if you have a heart condition.
4. Collapsing switches. Instead of sliding down into a contact, the switch collapses into it, like those collapsible camping cups. This is similar to a rubber dome though. These don't require keycaps.
5. Stabbing switches. They have a little needle in the center that pokes your fingertips when pressed. Requires special or modified keycaps. Needle height is adjustable and can be used to train the user not to bottom out. Gives a new meaning to "stabs". Also good for checking blood glucose levels.
6. Whoopee cushion switches. Each switch has a small bag of air underneath it that emits an imitation fart noise when pressed. It refills with air on the upstroke.
7. Kerchunck switches. Similar to a very tactile clicky switch, but it feels as though the key is actually actuating something in a machine. A true mechanical switch, akin to a typewriter switch. These can be made with levers or pulleys.
8. Musical switches. Just like a piano, each key strikes a string that emits a different note when pressed. Concert C pitch.
Someone get Kailh on the phone!
