Controller haptics for your hands contacting other avatars is a great idea, but let's take that even further:

Many (most) VR headsets have a field of view small enough that people standing to your left or right are not going to be visible to you. As a result, when they might be interacting with your avatar just outside your view, you are left completely unaware a majority of the time, especially if you have earmuffs enabled with a very narrow cone and can't hear them either.

To assist with this, let's use avatar touch haptics in a new way: When someone contacts one of your avatar's bones, or their capsule overlaps with your own, while they are not visible to you, vibrate the controller - left or right - on the side of you that they currently are, with both controllers vibrating indicating that they are behind you. Perhaps to distinguish this from other forms of haptics, it can be given a noticeable pattern, such as a heartbeat rhythm, with customizable strength, and the option to filter it to only friends if you want to prioritize them. Additionally, OSC hooks for this could allow it to instead activate other things, such as overlays, haptic vests, and more.

Overall, I feel this one small change could grant an extra bit of tangibility to the avatars around you, mimicking one's real-world sense of touch, letting you turn to see who just touched you. It would make interaction within VRChat feel even more personal and real to those who want it.