They are best in the more creature focused ones for the obvious crew reasons
Bonus sheets from previous sets sometimes have a unifying mechanical theme (in Outlaws of Thunder Junction , for example, all the cards from the Breaking News bonus sheet could commit a crime), but there's no specific mechanical thread connecting these Through the Ages cards
City of Shadows Currently for Sale Unfortunately, the same facts that apply to Goblin Wizard apply to City of Shadows
Artifacts / Colorless Spells Starting off with artifacts and colorless spells seems like the most obvious, as every deck runs them
Sadly, this card isnt a great Changeling if you have a Typal deck
And WotC has, in the last five years, made an effort to make sure each of their many Magic games receives enough pieces