If you could change Backbone at will, how would you?

Sooner or later, there will be a Backbone 2.0. Major releases are disruptive for users, so I prefer to make them as infrequently as possible. But when we do one anyway, we might as well use it to its full potential.

This is a "wild ideas" ticket, where you can suggest any backwards-incompatible changes that you would like to see in a future release of Backbone. Please keep it brief, but do explain *why* you would like to see the change(s) that you suggest.

You are also welcome to highlight anything that you want to stay the same. Again, please with reasons.

Some rules of the game:

- State your reason for wanting the change (or non-change)! It does not need to be a "good" or "smart" reason.
- All ideas are valid, bearing in mind that there are always tradeoffs. Please keep it very civil and do not enter competition with other suggesters over whose idea should "win".
- The fact that an idea is mentioned here does not say anything about its likelyhood to make it into Backbone, even if that idea seems popular. In the end, the decisions are made by the people who do the work, with me and @jashkenas having the final word because we are publishing the package and we need to assure its quality.

The following prior tickets are somewhat related: #4274, #4245, #3560.

Start by reading the issue's related tickets #4274, #4245, and #3560, then review the discussion for proposed backwards-incompatible changes to Backbone. This is a brainstorming ticket with no named files, tests, entry point, or concrete definition of done.

Written by the indexing model from the issue text.