Allow engines to be stacked lengthwise, to increase the power of the end engine.

[Mae!]

I believe that implementing this suggestion would significantly benefit players who hope to maintain the aesthetics of their ships. At current, if you want to make a racing ship, you need a huge, wide area of thrusters at the back of the ship that looks unpleasant. Stackable engines would allow sleek designs that are still functional.
To prevent capital ships from abusing this suggestion and making a thin single line of thrusters going through the centre for protection, the power added with each new thruster to the stack may have diminishing thrust returns, or increasing fuel inefficiency.
The thrusters would need to be physically touching at either end to form a stack. If one of the middle thrusters were to be destroyed, the thrust now being generated inside the ship would do significant damage to the surrounding area.

 

[WolfiAUT]

I think weight will be a big part of the game... so if your ship is small, you won't need as many thrusters.

I'd hope for some high performance thrusters for small vessels.

 

[Cavilier210]

I like nacelles myself, and so stacking would be nice. With limitations of course. The mad wall of thrusters on the back ends of ships can be real hard to manage aesthetically. Though, there are methods to accommodate the issue.

I do like the aspect you added about destroyed midstack thruster damage, and would support such a feature. If its feasible.

 

[Burnside]

I still want the tiered-up turbine-and-baffle megathruster concept for aesthetics (see below). Even using the straight turbine without plugging any other parts onto it, you have a larger thruster, baffling and input-thrusters just increase efficiency and accomplish the thrust-stacking concept nicely without making box and triangle thrusts some kind of ugly self-stacking blob system. With tiered plug-ins you have to design around the open spaces of the component systems and eat some volume inefficiencies for greater thrust potential.