Snapping to the ruler is also very nice, they sort of work like guidelines in that respect. Yes, that's it: see it as short guidelines that are temporary :-) (one shouldn't save them in the file for example)
In many 3D mechanical CAD programs they have something similar, called "construction lines" (for example in Unigraphics). These lines are only used to construct sketches,
In place of this I often use the trim/extend feature. After I dont need the line anymore, I simply delete it. This method is simple, and straightforward.
I never really understood the goal of the construction lines in autocad, when there is simple and easy replacement. (just drawing a line and extend it)
As for your proposal Max, I think any tech drawing related improvement is useless without *numerical input*. So defining a circle through three point is usefull only, if you can specify the three point numerically. If it is not the case, it is only a yet another artistic tool.
I cant say anything as developer (because im not), but as a user, I would be really thankful if you choose the tech drawing proposal. I really miss some tech drawing capabilities from inkscape.
Anyway, sorry for the noise, and best wishes for your proposal!
Best regards, Khiraly