Here are my ideas for what they are worth.
To start with I haven't gone down the route of trying to combine Steck and System-X as I think they are two different to be made compatible in any real sense of the word, so I've gone the route of a new system, or maybe a similar system to Steck is nearer an accurate description.
The flexibility of a construction system is directly related to the smallest unit of the system. The smaller the unit, the more flexible the system. At one extreme, a system such as Lego is very flexible because the basic brick unit can be combined in lots of different ways to create buildings of different sizes and designs. At the other extreme, a system where there are, for example, four walls and a roof which can only be combined in one way and cannot be examined is quite inflexible. I've gone for the idea of two basic units, a full wall and a half wall and a range of connectors to allow different shapes and angles in a building.
The basic wall and half wall units can be produced to different designs, e.g. sold walls, walls with doors (square topped or round topped doors, single width or double width doors), walls with a window or windows, walls with different styles of windows, e.g. house size or shop size, walls with a door and window combination and so on.
There is further flexibility in that the wall surface can be varied, smooth, brick, stone, timber and plaster and so on.
Attached are sketches of the basic wall and half wall design. Construction is by the tab and slot system, similar to Steck. I've shown two tabs, but there could be more, although I think not fewer. The dotted lines on the left hand side of the unit indicate the slots for the tabs for joining on another unit.
The dotted lines on the top and bottom indicate slots to allow vertical connection for two storey buildings or for a floor or for fitting a roof unit. The connector would be a cross shape.
I'm off now to sketch the connectors to allow corners and joins at an angle other than 90 degrees.
