Ideal Features of Online Homework Platform?

If you could build an ideal online homework platform for mathematics, what combination of features should it have in order to be effective at levels from high school through undergraduate, and why? Please post what features an ideal, effective online homework platform would have for use in math class at a high school to undergraduate level. Ideally, you would support why those features are important, with some theoretical (education theory or mathematical) or practical reasoning.

This question is not asking about whether online homework or paper homework should be used. Nor is it asking about existing homework platforms. I think that this makes the question unique on this site.

I am not posting this for my own market research purposes – I think it is fairly clear from my history here that I teach classes and do not have any incentive or energy to create such a platform. Instead, noting the glut of bad online platforms, I hope to combine our collective knowledge here in a publicly visible arena, in hopes that people who do attempt to create these platforms have an easily accessible list of must-haves, with at the very least keywords that point towards research support.

Best answer will be awarded to the most complete list with meaningful reasons for each item. I do plan to answer this question myself when time allows.

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Some example features that are in my personal list of ideals, with my reasoning being "practical purposes" and/or research-supported tactics italicized:

• Must be able to select problem sets from an easy-to-navigate problem bank, for practical purposes (teachers should not spend too much time searching for problems to assign).
• Must be able to randomize the numbers in each problem, for practical purposes (students can work together on the same problem but still have to do their own work).
• Should identify commonly-confused concepts and have pre-made problem sets contrasting the types of problems for interleaving purposes.
• Should make it easy for the teacher to promote spaced repetition (e.g. when creating a problem set, it could auto-suggest problems similar to two problems from the previous week and one problem from two weeks before).