If instead the creature you control is an illegal target but the creature you don't control is still a legal target, nothing will happen when Rabid Gnaw resolves
Sometimes its just Ashes of the Fallen and Return of the Nightstalkers
If Surge Engine loses defender in some way other than its first ability, its second ability may still be activated
Cards are usually not banned for power level reasons, and the last time it happened was Lurrus Of The Dream Den prior to the nerf to the companion mechanic
Choose your fortress, secure your cards, and hit the local game store with absolute peace of mind.
If your deck's key engine is a 4cmc commander like Kaalia of the Vast, for example, then you will want to run ramp cards that can get her out faster, which means ramp that is between 0-2cmc so you can cast Kaalia on turn 3 or earlier instead of her usual turn 4