Your suggestion would help to at least filter the threads displayed to only baseball related ones. There is a link on the left side of each set's page to any Forums that have been posted for that set. Unfortunately, not all member's who post about a particular set actually uses that link.
The Forum Search function does help but because threads are poorly titled in many cases, the search for a particular topic may not find everything you wish to see.
If you want to have a drop down menu created, suggest a New Feature. You can find this under "Things to do" at the bottom of any page. It is listed in the column titled "Features". Once on the New Feature Tracker page, there is a "Submit" button. By clicking on it, you can describe what you are suggesting and a short explanation of why it would be useful. Admin will consider your suggestion and if he thinks it is worthwhile and can be put in place, he will list it as an active New Feature that other members can vote on.
Bruno
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Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0