5 really, though two are minor tweaks to dependent on other changes.
Mainly just time/space/material/headache savers, as everything is doable currently without these tweaks with various hatch/plate/pump setups.
1: Reverse linkage designation. If a bridge/floodgate/ect... is set to reverse linkage (set when you create the link) the object will receive the opposite command it normally would. So if you have a floodgate and a switch, and link it as a reverse set, when the lever it pulled to the on state, the floodgate will close, and when the lever is pulled off, the floodgate opens.
2: Pressure plates (and other triggers) can link to and change lever states. Simple and straightforward, save mechanisms, even more useful with 2.a and 3. Example: 2 pressure plates to 1 lever, lever to 10 spike traps in hall, if something sets off your pressure plates it triggers the lever, which in turn triggers the spike traps, 12 links instead of 20.
2.a: New command in switches, lock in place. Only relevant with the above change, locks a lever state so changes from other triggers linked to it get ignored. Example: As above, but something shows up you want to let through to your cage traps instead of impale, so you set the lever to lock off and the pressure plates get ignored (just remember to unlock or you security remains disabled).
3: New trigger designation, manual reset. Between resets and one use, a manual reset trigger will send on-state commands and resets itself after use, but doesn't send off-state commands. More useful with more trigger types. Example: Pressure plate (manual reset) to lever, lever to drawbridge, creature triggers pressure plate, which triggers the lever, then on to raise the drawbridge. But drawbridge doesn't lower even when the creature steps off, drawbridge only lowers when dwarf pulls the lever back to off, then next time something steps on the pressure plate it raises again.
3.a: One time use triggers only require one mechanism to create a linkage. 1 mechanism in the trigger, making the linkage job faster and gives you a chance to save the mechanisms in a deadmans trigger scenario.