What I'd like to see is a more variable outcome when attempting a bribe.

If the bribe is accepted, it shouldn't mean that the entire stack simply deserts: some might stay loyal, others switch sides or become rebels and some might just disperse. The chances for each would depend upon culture comparison, whether the units are elite or militia types, whether the factions are at war, whether they are in the army's or diplomat's territory, and the size of the army, since a lone unit would be far more likely to accept a bribe than one surrounded by potentially loyal witnesses.

Another change I'd like would the chance that unless the troops completely disperse or desert that they kill your diplomat, even if he's a 10 star and they're peasant slaves. There would also be a chance that they'd take the money no matter what else they do.

Bribery could thus still be used to disrupt an enemy, without so much certainity as we have now. Instead of the current dull process of just handing over a bag of cash and yawning at yet another failed invasion attempt, you could have the fun situation of ending up with a dead diplomat, stolen money plus a 3-way fight as you command the deserting troops against the no-doubt furious loyalists, with other troops that decided to turn brigand complicating matters. Even the latter outcome might be worthwhile though, giving you enough time to move up an army to intercept any remaining enemy troops.