The classic design trade-off is probably retractable landing gear. Over the years I've noticed pattern planes add retracts over fixed gear only to shed them a few years later. It would seem the lower weight (and thus inertia) makes the plane more tractable and reactive to inputs. (Changing the direction of a rolling basketball is easier than a rolling bowling ball even though its larger.) I've also been told that a dirty shape helps prevent the plane from accelerating in the down-leg part of a maneuver where constant speed is required.

While the retracted gear gives the plane a higher maximum speed for a given engine power it's probably only important if you have a high priority to go FAST. Parasitic drag starts low and climbs quickly with airspeed because of it's geometric relationship to velocity. Inertia, on the other hand, is directly proportional to acceleration so can effect performance over the entire range of flight speeds including a slow landing on a gusty day.

With that said, I think everyone that designs their own planes wants to build a rocketship at some point. I'm currently working on a twin that has retracts, lengthened nose for better penetration, streamlined nacelles with minimum frontal area, large transition fillets between wing and fuselage. Will it go fast? Man, I hope so....It's been a LOT of work and $$$.