Once you have sizing, the ability to make an issue dependent, estimated start / end, don’t you have everything needed to generate a PERT / Gantt chart?
You also need the proper dependency types[0] I mentioned, though I'd say you can get away with FS for dependencies and FF for subtasks.
Beyond merely generating a Gantt chart, when you have dependencies and estimated duration you can start estimating how long things will take and which ones can take longer or be delayed without impacting overall project time; add the ability to provide constraints on start and end time, and you can automatically schedule work (and re-schedule at it happens).
Most of these things are in every system in some form or other, but dependencies seem to be missing almost everywhere - and dependencies are the one key component that enable major benefits and turn the system into a proper project planning tool.
