If we ignore the idea of a challenge system completely (which I do think is a legitimate option, but will drag the thread off the rails).
The next most obvious set of improvements is in the was TMO is applied in rugby (Or at least, was applied pre-bunker review, I think that's a backwards step).
* The process can be initiated either way, by a TMO who thinks the referee has missed something or by the referee knowing that he's made a call but he's not confident about it.
* Review is carried out on a big screen (where available) so everyone in the ground can see what's going on
* Audio can be broadcast live, both in the ground and on TV so the discussion and the reasoning is clear
* And once a decision is reached, part of the process is explaining the decision to the relevant captain(s), so that players are also aware, before any card is issued or restart is taken
That process is helped in rugby by decision flow-charts that have been shared on here before, and clear understanding of what can qualify as mitigation and how a decision can be modified if there is mitigating circumstances. (My recollection is that this topic in particular came up around the Liverpool Jones red card review vs Spurs, where no one seemed to have any idea if his foot bounding up off the ball was allowed to mitigate the severity of the sanction or not). So there's definitely some law improvements to be made, to allow more consistent use of VAR and to create a structure to the discussions. But day 1 easy improvement is opening up that decision process.