at their own 35 yard line with a 4th and 10 (or 15). They'd have the option of punting or going for it. This would replace the kickoff with a typical punting situation, where you don't have as many gunners running free down the field.

and it is intentional to allow the defense to get positioned before a returner can build up a head of steam to hit up the line at full speed.

The kick-off is a live ball in both sports. So an up-and-under style of kick also acts like a version of the onside kick. The receiving team has to make sure they get to the short kick first. Where would you deploy your return man if the kicking team varied the placement of the kickoff much more than they do now?

In general, kicking strategy in rugby is more interesting than in football, but there is nothing in the rules preventing football from using the rugby strategy.

You can close those 20 yards pretty damned fast.

Fair catches on short, high kickoffs would kill excitement, but removing fair catches (and "kick catch interference," I guess) from kickoffs might make it more dangerous again. For those reasons, I don't see rugby-style kickoffs entering American football right now.