I think Engi could use different types of sentries, like a rocket sentry or a flamethrower sentry. Rockets move slowly, but deal as much as damage as Soldier's rockets do. Flamethrowers have a short range of fire, therefore best when built in corners or narrow spots. Ice sentries and electric sentries would be also cool. Both have a short range of fire, by the way.
Ice sentries deal no damage, but greatly slow down the target for 5 seconds or so. At this point, they take twice as more damage from all melee attacks. The target will be shattered into pieces if killed by a melee weapon. Also note that pyros can thaw their teammates, but not themselves.
Electric sentries fire a "thunder" on the target. They deal very small damage (-5HP or so), but the target, due to the shock, will be unable to use any weapons for like, 4 seconds. However, perhaps due to the adrenaline boost from the shock, the target moves slightly faster until the shock wears off, enabling them to outrun their enemies. So, scouts won't have much trouble with these sentries.
Only one of these sentries can be equipped by the Engi at a time, alongside the normal sentry. As a downside, equipping one of these might have Engi carry less metal or something.