You can change a filter predicate dynamically - so if all your data were in one table and you had a column for the Region, you could do:
Items: Filter(tblSales, SalesRegion = varSalesRegion)
You cannot specify the table with Sales_ & var_Reg. PowerApps evaluates everything 'on the fly' for the App Checker and intellisense won't be able to figure out what datasource you want and so what fields should be listed unless you explicitly specify the table you want to use.
You might want to try, in the onchange of your first combo something like:
If(First(cboReg.Selected.Items).Reg = "SouthReg", set(varReg, SalesRoute_SouthReg))
Where SalesRoute_SouthReg is actually the table.
And then just use varReg for the Items property. I have one similar for the Items property of a Datatable but that was just for a quick 'utility' that only I was ever going to use - I wouldn't trust that in a Production system.
I would be especially wary of using this technique with combos as, in my experience, they are quite finicky about the data source and if you change it, you might find you lose your parameters for the Display and Search fields.
Safer would be to have multiple combos, one for each region, and base their visible property on the chosen regions.
Better still would be to normalise your sales data so that it is in a single table with a field for 'Region' (instead of being split across multiple tables, one for each region) and then use a filter to get the appropriate subset of data.
