Well, rather than give the game away completely, and since I've commented recently on another arm design, here's what I wouldn't do.
If I used ball races, which I might do, I wouldn't align those for vertical movement in any way other than to have the cartridge move vertically, in the plane of the cantilever, and the counterweight parallel to that.
The axis of the bearing for vertical movement would not intersect the arm tube axis, or be above the stylus.
The axis of the bearing for horizontal movment would not intersect either of the above axes.
The counterweight would not be on the same axis as the arm tube.
There would be no arm "tube".
I wouldn't have a fixed geometry, as all parameters would be adjustable.
I wouldn't omit anti-skate.
The arm wouldn't be symmetrical
I would avoid manual adjustments.
I would not have just a 9" version.
As far as the old RP1 is concerned, the only things easily changed without altering it too much would be:
1. to provide a sliding base with better adjustment of arm pillar height (an adaptor for Ortofon arms which is now available on ebay will do the job)
2. redesign the headshell to provide variable offset (which I did on the last few arms)
3. provide a secondary counterweight for adjusting downforce
4. maybe a secondary counterweight to improve lateral balance
