Create a distinction between low-stress exercises and high-stress exercises. The former can be done to failure; the latter have to be stopped as soon as your form starts to break down. The former include a lot of body-weight exercises - push-ups, sit-ups, flutter kicks, planks, etc. The latter is any exercise where a breakdown in form is actively dangerous - deadlift, snatch, clean and jerk, squats, and so on.

Do the former all you want; the worst you're going to get is some soreness and maybe muscle strain. The latter will tear things.

"How would you modify crossfit to make it safer and more effective and keep it fun"

But in that case, it probably wouldn't be Crossfit and maintain the same mass appeal.

Nothing but bodyweight exercises to begin with. 100 burpees will get you ripped and even grandmothers can reach that goal safely.

Just listen to your body, if the weight is too much remove, if the reps are too many take your time or do less. If you want to show off or feel peer pressure then you shouldn't be crossfitting. I am doing CrossFit for the last 6 months and I always stop when I feel uncomfortable or in danger.

sure, you could modify crossfit to make it safer and more effective but then it wouldn't be crossfit, would it?