Problem with that is it could reasonably be expected that it would provide an unfair advantage to clubs playing home games in the warmer states, clubs playing home games on smaller grounds or clubs playing home games in a dome. It may also create an unintended consequence at the end of the season where teams go ultra defensive to prevent the opponent from securing the bonus point.
It's also problematic with the uneven fixture when you have a side like Essendon in 2016 (they weren't even the poorest defensive side that season: Brisbane somehow averaged 130.5 points against them) or expansion GC/GWS: we get to play them twice and now you're practically gifting us a bonus point too?
A point for each game over the score is far too generous too: maybe something like a point once you do it three times would be better.
I’m not worried at all about the unfair advantage side. There’s so many other quirks and unique feature of the fixture and it just sits alongside Geelong’s home double home games, the extra travel some teams do vs genuine home advantage, etc. I honestly don’t think there is that much evidence that teams score more highly At certain grounds - even wet weather has less impact than it used to because so many scores come from turnovers anyway. Expansion is a once a decade type thing, and honestly, in the current system you already get a ‘5 point’ bonus playing a team like that because of the percentage boost.
And ultimately, the purpose of the competition is entertainment, NOT fairness. Something that confers a marginal, indeterminate advantage to some teams, possibly, while making the game lots more entertaining, is totally fine with me.
I do worry a bit about unintended consequences and late season shenanigans though. It opens up for some silliness with tanking (teams deliberately not kicking going for 100 to maintain draft position) or manipulating finals opponents. But until it happens, that’s also a long bow, and it’s also not a reason not to do it.
I’d be ok with a bonus point for every 3x 100 points, although I don’t know that solves either issue you raise, and probably removes a lot of the incentive to target it, particularly at the top end of the ladder.
Ultimately, it makes far more sense that trying to change rules around the ground. The holding the ball mess for example - it doesn’t seem to reduce congestion, has just led to more sloppy play and fan annoyance, confusion, because ultimately it does nothing to change the incentives of teams or coaches. A bonus point for high scoring does, a lot, and that’s why it works.