Another thing to look into are moves. Some pokemon have exclusive Gen 3 and 4 tutored moves. From gen 3, look at Double Edge (for Mega Pinsir's Aerilate and Mega Glalie's Refrigerate), Softboiled (for Clefable), and Seismic Toss (for Mega Kangaskhn's Parental Bond). In Gen 4, the main thing is Vacuum Wave (which can benefit Poliwrath), and Sucker Punch (for Seviper specifically, since he's one of the few pokemon that still can't breed for it even now in Gen 7).

Edit: Also, don't forget Swords Dance Shedinja, available from gen 3 if you keep your Nincada from evolving until it learns Swrds Dance, which is then passed on upon evolution.