Alloys/metallurgy : promising elements for for Sn-based solders

I am trying to research potential modifications to lead-free solders (intended to be used in electronics). Base alloy is eutectic $\ce{Bi_{58}Sn_{42}}$, and I am going to add various combinations of a bit of copper, silver, gold, platinum, gallium, lithium, and indium.

Is there something there which is known to not work? I am wondering this because I see that gold, platinum, gallium, and lithium are not typically used as dopants for electronic soldering alloys. Should I expect a lot of problems attributed to intermetallic compounds? (I know there are issues due to a Sn-Cu reaction, but nothing about the rest).

Is it theoretically possible to find a combination with significantly high copper content (> 10-20%) while still having low melting temperature (< 250 $^{\circ}\mathrm{C}$)?

phase diagram "copper-indium" alloy