Rethinking the nickname requirements for participants

Is your feature request related to a problem? Please describe.
Right now Decidim requires a nickname for every participant.

Based on our experience it is unclear for users why they need to fill a name and separately a nickname. Most users don't understand what this nickname is for.

Another issue with this is related to the 3rd party authentications that generate the nickname based on the user's name. This way user names are leaked through the nicknames, even if the user understands to change their name.

Some users have also shown their concerns that their email address is publicly visible on the platform because the nickname can be mistakenly seen as an email address due to the "@" character included in it. Not everyone understands the purpose of the nickname and what it really is.

Describe the solution you'd like
Decidim's nature is more like a community platform where users would present themselves with their own names. People are more familiar with platforms such as Facebook where you do not need a nickname and very few people of the target audiences are familiar with platforms such as Twitter or Reddit where nicknames are used.

This is of course problematic for referencing other users on the platform e.g. at private messaging or in the comments because many users can have the same first name, for instance. But perhaps there could be some other way of uniquely distinguish the users from each other, such as the user ID.

If the nickname is seen as an absolute requirement, keeping the manual registration process harder, I would suggest that at least the algorithm used for automatically generating the nickname during 3rd party authentication would be changed. We could either use arbitrary database of words and create the nicknames based on a random word pair from those ones. Or alternatively generate nicknames such as "user123".

Describe alternatives you've considered
I have described two separate alternatives in the description. And another alternative would be to use some kind of completely anonymous platform where users can act without a public profile created about them.

Additional context
Add any other context or screenshots about the feature request here.

Does this issue impact on users private data?
Yes, this has a positive impact on user privacy, by not leaking the user names through their nicknames.

Funded by
We can help developing the solution for this once agreed on the approach but we cannot fully commit funding 100% of it, because the development approach also depends on product decisions.