You should be able to change the minimum roads/knights for the Longest Road and Largest Army cards similarly to how you can change the card limit to get robbed or the amount of vp's to win a game.
A game with minimum 2 road LR and/or minimum 1 knight LA would be pretty interesting such that the first person to place a road or use a knight will automatically have it and it'll be contested throughout.
Furthermore, it'd be useful for balancing other sorts of gamemodes. I've been trying out 4 vp games and they're a fun and interesting change of pace, but if I could change the road minimum to 6 or 7 it'd be far more balanced.
Also, it's just a pretty common sense addition. Catan is always a game about changing things up each time, so this just seems like a logical next step