I mean plain 68000 (not 68020, I have never examined/played with anything 680x0 above 68000). I'm not talking about indirect calls/jumps through aN, but through memory locations. I don't remember below instruction to be possible/supported on 68000, is it ?
For a simple 68000, you are completely correct. For the 68020, there is the double-indirect mode which allows that if combined with the suppressed base-register and long displacement.
bsr (funcptr)
'funcptr' being label for some (relative) address.
Well, indeed, but that's more a restriction of the "branch" family. All "conditional branches" only take byte, word or - with the 68020 - long displacements, but no directions or
-calculations as the jmp/jsr family does.