In my view, SO sites work best when there is a clear focus - i.e. the topic is well-defined, so people can easily tell whether
- their question is relevant
- the site caters to their interests
- a posted question is on- or off-topic
The advantage of "Lego" as a topic is that it provides that clear focus straight away. However, I think with some work it should be possible to expand the topic while maintaining focus (with the advantage being that more people with the right mix of interests would be attracted). But it would be necessary to draw the boundaries quite carefully. For instance: what sorts of building materials would be covered? Lego, clearly; Erector Set (=Meccano)? Capsela? Sticklebricks? Balsa wood?
The definition would either need to list the types of material explicitly (e.g. Lego+K'nex+Erector/Meccano, but nothing else), or give a definition that sets out what is covered (e.g. sets where models are be constructed out of pre-made pieces, without cutting or gluing).