navigation – I’m not sure what to put on my desktop app’s home screen

I’ll take your word you cannot predict the sections users are likely to want when the app starts (but see jgthms’ answer before settling on that). You’re correct that if you already have an efficient sidebar menu on every section, then it doesn’t make sense to make the home section the “speed dial” page. Eliminating the sidebar in favor of the home speed dial controls slows navigation, and having both adds complexity with little benefit.

Enhanced Navigation

However you might be able to do something to enhance the performance of the sidebar menu on the home section. For example, if the side bar menu has cascade menus, then the home section can display the cascade menu items across from their respective sidebar menu items, as if they’re “pre-opened.” That will give the user on the home section one slew-and-click access to any menu item, while still being consistent at the top level with the sidebar menu for every other section. Such a home section also educates the user of what options lie under what sidebar menu item so they can find them faster when using the sidebar menu in other sections. A page with all menu items visible is faster to use and less disorienting than a cascade menu (see Bernard ML, Hamblin CJ, Chaparro, BS (2003) Comparing Cascading and Indexed Menu Designs for Differences in Performance and Preference. Proceedings of the Human Factors and Ergonomics Society 47th Annual Meeting, 1370-1374). It’s just we usually don’t have the space for it, but here you do.

As similar example, the home section could include controls to set the parameters for each section. If the user needs to filter or populate the content of each section (e.g., provide a date range for the Orders section), you can list the most recently used date ranges across from the Orders menu item on the home section. Or you can provide blanks to enter the date range on the home section so the user can get to a populated Orders section directly.

Work At Home

You already have notifications and alerts on the page, which are presumably important to see when the app starts (maybe even more important than the navigation options). Is there anything else you can put there that useful to the user? Maybe the home section can feature subset of the content of each section (again, proximal to the corresponding sidebar menu) that allows the user to do the most common and easiest tasks for that section right on the home page. Maybe some users will never have to leave the home page, making for simpler and easier-to-use app for them. If the users find they need more detail, they can “drill down” to the section, which will carry over their input from the home section.

Nothing

Your concern is there’s “not enough to fill the screen.” Why is that a problem? Blank space is a pretty good indication to the user of what they need to do next (create or retrieve something). Many office-type apps start with a blank virtual page. If users are complaining about it looking too boring, then maybe add some non-distracting wallpaper. Or make the window smaller (e.g., tall and narrow so it’s tight around the sidebar menu), and expand it only when the user selects a section to show. If adding anything else to the home section just adds useless clutter, then maybe your job is already done.