The most common noun ending to indicate a profession, a special capability, a philosophical leaning, or the like, is -ist (e.g. chemist, hypnotist, socialist, etc.). There are literally hundreds of such English words, far too many to ask you to try to list all (or most, or many) of them. Here are a few other endings that serve a similar function, but where it is reasonable to attempt to list many of them.
Certain scientists, medical specialists, and others are indicated by this ending. How many can you think of in half an hour? (18 = good, 23 = excellent, 25 = extraordinary)
There is often, but not always, a negative overtone to professions or specialties or other activities having this ending. How many can you list in half an hour? (15 = good, 18 = excellent, 21 = extraordinary)
This ending narrows the –ist list considerably. (Most but not all are specialists in a corresponding –ology, and not every –ology has an –ologist.) How many can you come up with in 30 minutes? (48 = good, 54 = excellent, 58 = extraordinary)
Some of the words you may have thought of for –eer are spelled with -ier, as preferred in French, and a very few words can end with either spelling. How many –ier words can you find in 20 additional minutes? (5 = good, 7 = excellent, 8 = extraordinary)
A much smaller class than the –ologist words, usually from fields that end in –alogy instead of –ology. Which ones can you think of? (1 = good, 2 = excellent, 3 = extraordinary)