- Hastening adds to init but gives you a penalty to rolls for one round
- We've never seen anyone actually use this
- Leadership skill boosts one other character's initiative for a round
- Also doesn't see much use
- Recoil penalizes your init next round for using autofire this round
- Combat reactions (dodge, parry, dive for cover) penalize your initiative for next round
ACKS has a simplifying solution for once: reroll initiative every round. Keep track of your flat initiative modifiers (Dex+Tactics). Over each round, keep track of your initiative debt via poker chips or a d6 that you can increment. At beginning of new round, roll init, add flat modifiers, add temporary modifiers (and clear your counter), and then the DM does the initiative countdown.
This also addresses another issue with Traveller's autofire and combat reactions: when used by characters with very high dex (as PC gunslingers tend to have), small init penalties often don't matter, because they're dominated by dexterity. When they do matter, the consequences (acting after a particular enemy) are usually perfectly predictable, and then they can be avoided. Rerolling initiative makes modifying your initiative an informed gamble, an interesting / not-trivially-solved choice. It also makes combat messy and chaotic.