doing 3d cad online (what do you think)

That is what tinkercad.com is, they just closed down.  There is a hole in this market.

3dTin.com and Autodesk 123D Make are other Saas options in this area. 

I'd easily pay $5/month for a new tinkercad-esque offering.

I agree- TInkercad started out as an entry point online software for non-professionals to learn the moves. Sad to see it turn its ways and hock up the price out of the hobby range. Now it's gone, presumably to be replaced by an even more expensive option. Not online, but http://www.freecadweb.org/ and http://thancad.sourceforge.net/ are where I am looking. Blender is a bit too intense for me just now.

@dubbsd

Are you looking for a website, or do you want to know whether the idea is good?

As for websites, I think the others have giving a good overview of current available options. If you're thinking about launching such a website, you need to think about what sort of modeling is (a) easy (nurb vs. CAD vs. sub-d modeling) (b) will make useful things (building cubes and pyramids will eventually make people bored) (c) technologically feasible and (d) will produce printable objects (presuming you want to use the website for 3D printing) and (e) will product reasonably priced printable objects (if you are thinking of printing your objects through a service bureau like Shapeways).

As for your monthly/annual fee, you should ask who this would speak to. In my experience, people who usually pay monthly/annual are either (a) companies/schools (at which point you have to ask whether they would just go for more expensive and powerful software like SolidWorks, AutoCAD, Rhino, etc.) and (b) people who use the software all the time (eg Dropbox, in which case you have to ask yourself what will keep your customers coming back).

I've been thinking about TinkerCAD since they shut down, and I have a difficult time figuring out how they conquered each of the areas I mentioned above.

Is this the feedback you are looking for?