The problem with calling it from a formula like SPARKLINE is that especially for progress bars, you want to let the user enter a percentage manually with the % clearly displayed in the cell with the bar in the background. If you use a formula, then you need two columns: one for data entry and a second for displaying the bar with the formula.
You could certainly create a custom Data Type specifically for progress bars, but that would be limiting the use. If you can control the Min and Max range as well as the number format (50% or 500 or 5.05 etc) then a “Data Bar” data type could be used for much more than a progress bar.
Custom number formats are powerful, so I hope that becomes an option at some point. Basically, anywhere you might be able to specify the format, a field for specifying the number format would be useful. And if you can use the same syntax as Excel and Google Sheets, that should help with compatibility. OR, maybe it would be easier to create a new Data Type called “Custom Format” where you are allowed to enter the syntax directly. Here’s a blog article that shows various uses.
Supporting a SPARKLINE function like Google Sheets would be useful, of course. You can create a mini Gantt chart with the SPARKLINE function (the stacked bar chart). This would be useful in a project planner as a condensed view of the chart, and could serve as a very useful placeholder until a new Gantt Chart view becomes an option.