When I play Boggle with my family, my mother the nurse is always trying to sneak in medical terms, my dad uses obscure engineering terms, my husband tries to use fantasy novel jargon, and I of course use the occasional French word.
While for the most part these words are found to be invalid for Boggle, it's still fun to find and share them. And, if you have more than one Boggle player with one of these inclinations in common, you could play a quite amusing game of theme Boggle. The key is to have more than one Boggle player with the same background so that they can police each other! After that the Boggle theme play possibilities are endless.
While finding foreign words in Boggle is not always an exact science because Boggle does not include the accents and other punctuation that are sometimes found in other languages, foreign language Boggle can still be quite entertaining. Usually the best way to play Boggle is to include words in a certain language other than English, but also to use regular Boggle words. It's not always easy to find that many foreign words in Boggle!
To satisfy the nurses, engineers, or fantasy nuts out there, theme Boggle may be just what the doctor ordered. If you can find coworkers to play Boggle with, at lunch for instance, then it's definitely worth making occupation-related words valid Boggle words. Doctors can identify names of diseases in Boggle, while computer techs use technology terms and software acronyms for Boggle.
If you're into history, it could be fun to look for historical people or places in Boggle. Or if you love reading fantasy novels, how about looking for those imaginary fantasy words in Boggle, like "ewok" or "orc?" Gardeners could search for latin flower terms in Boggle. These are probably not permanent ways to play Boggle (particularly since you need to play with like-minded people), but theme Boggle versions can be fun to shake things up a bit.
For a brief break, try playing Boggle where any word you can justify as having meaning in some way is valid. It's best if Boggle players try not to be too competitive for anything goes Boggle because that will likely just lead to frustration. As a sometimes Boggle purist, I know! But this kind of extremely loose Boggle play can be a silly opportunity for players to explain why, "hogwarts" or "cowabunga" or "AMA" are valid. It's like the Boggle version of Balderdash!