If you have traits you wanted on your main heroes now you do have 3 possible options in my opinion:
1. You will take care of traits of other important heroes needed for some of other modes, and when needed you will have more good options to pick from,
2. You will use your gems to get something else from different event which are during "Treasure Mining" event like Tree of Choice for example searching for more resources/ coins/Magic Powder, etc,
3. You will save your gems for new "spending" events which will happen sooner or later, and thanks to that you will have more options in those events.
I don't know what "main heroes" you are talking about, but as for me i would keep adjusting traits for "main" and some of other good important heroes which are really needed in certain modes like Dungeon/Team modes/Archdemon etc. while trying to get more coins and magic powder. Those 2 are really good for everyone, and you can never have enough of them.