Depends on size and budget. For starters, have a look at the Silverstone SG13 case. Image is an old photo of a build in progress for a taste of the insides. You can go Intel or AMD, pick a lower power CPU, enough room for a 120mm AIO cooler although air is possible too. This is quite comparable to the old Shuttle boxes and you can pick whatever spec you need. Note on the case, it comes in mesh and hard front versions. Get the mesh one if you want any airflow at all in the case to help keep things cool. Not shown in pic is a regular size ATX PSU fits in there so no need for premium smaller ones.
If you want to go smaller than that, have a look at NUCs. Upgrade options are more limited there but 16GB of ram should be possible.