I like the simple tree-like hierarchy structure initially.
In addition to that, being able to "tag" outcomes by grade level would be useful.
The grade level isn't always part of the tree. For instance, US career and technical standards are not grade level specific - they are for all high school students, so they could all be tagged with 9, 10, 11, and 12. Whereas national science standards, even covering similar topics, have separate standards written for each grade level. One might want to sort by a specific topic, and ignore level - for instance, a teacher teaching many grade levels. I am an agriculture teacher, and teach students grades 9-12, but I might be interested in viewing lower-level math standards by their topic areas. Even though measurement conversions may be learned in elementary and middle school, I see value in reinforcing and practicing that at the high school level, so I might want to ignore grade-specific aspects of a tree structure when searching a standard set in Math, for instance. But some teachers who only teach one grade may want to sort out all standards so that they're seeing other subject areas beyond their own even, but only for that grade, so they have some idea of what their students are learning in other classes.
Having the grade level be part of the tree structure would require making a "duplicate" of those standards in each of the grade levels it is part of - and that might be 13 duplicates that aren't actually connected, in some subject areas.
It would probably be good if institutions could add their own grade level options, too. For instance, a middle school may only want to include grades 6-8. Or, the entire school district may have a Moodle and want pre-K through 12. Or maybe, a school has an articulation agreement with a local college and offers some courses for college credit, and/or high school credit. So some "odd" grade level might be required, such as Grade 13 at any university. Or Grade 13 - ABC Institution only. Etc... So the grade level options should be editable.
This would also help teachers to facilitate looking at how "well" a particular student reached a particular outcome in a grade-specific course, and they might know that that student may need extra practice in a certain area in their grade level if outcomes are also tied to specific users. Teachers of different subject areas would benefit too. For instance, sometimes as an agriculture teacher, I'm not familiar with how much all my students know about graphing - parabolas, sin waves, etc, and I need to talk about those types of graphs in a wildlife ecology class, but it might be useful to know which students have that subject down, and which never elected to enroll in Algebra. Or, being I have a wide age-range, sometimes geometry concepts are familiar to some of my students, and completely new to others.
So also that idea of having outcomes being linked to a user - and somehow even when the same outcome may be used in different subject area by different teachers - would be a neat tool if it could be implement.
