This is some ideas I have for jetbrains space, to be clear, I use Jira enterprise primarily and these ideas are what I use most on jira and in my eyes, needed for me to even consider paying for jb space.
1. Github, gitlab, bitbucket and other 3rd party VCS intergrations. I use jira, but use github for our VCS, having unbais support for the development tools i choice to use is absolutly required. This will make sense in the next suggestion.
2. Project workflows, Jira workflows are great, when configured correctly you bearly need to touch jira apart from to look and track issues. I have projects setup so so that and issue in the backlog once someone moves to in progress Jira will automaticaly assign them to the issue and start time tracking. After this, pretty much everything is automated. Once the branch is created, jira links the issue to the branch so anyone checking the issue on jira, can click on branch and be taken to the branch on github. Once a PR is open on github for that branch it is jira moves the issue into IAT, once it is merged it is moved to waiting release. Then i have webhook setup in our deploy system once the masterbranch has successfully deployeed to our servers jira will move all issues that are waiting release into done.
There are other workflows as well, I have one setup to send an email to the developer + management if an issue has been in progress for over a week and alot of other small easy of life workflows.
3. sub issues. We do alot of new projects because of this we often use trees to detail features/bugs/systems For example a root node my be "A user can have a user profile" then that would have sub issues like "a user can upload a profile image" or "a user can have profile comments" (these also have workflows where once all the sub-issues are done the root node is also moved to done) Its a sort of BDD process. From what I know, you track has this, (but I dont like youtrack its too, clunky). I could see checklists doing this sort of feature, but they cant be tied to an issue at the moment, maybe add a way to link a checklist to an issue?
There will prolly be more suggestions once I think of some, but these are the major ones which stick out to me.
On a final note, I get what space is trying to do, it trying to shake up atlassian, but even i know rome wasnt built in a day. the feature list space has planned verus what its got is astounding.