MyCommand[
    opt1="this is val1",
    opt4="this is val4",
    optN="this is valN"
]{argument1}

MyCommand{
    arg1="this is val1",
    arg4="this is val4",
    argN="this is valN"
}

documentclass{article}

usepackage{xparse}

ExplSyntaxOn

NewDocumentCommand kvargcommand { +m +m }
 {
  omnesia_kvargcommand:nn { #1 } { #2 }
 }
cs_new_protected:Npn omnesia_kvargcommand:nn #1 #2
 {
  group_begin:
  keys_set:nn { omnesia / kvargcommand } { #1 }
  cs_set:Npn omnesia_action:nnnnnnnnn ##1 ##2 ##3 ##4 ##5 ##6 ##7 ##8 ##9 { #2 }
  cs_set:Npx __omnesia_tmp:w
   {
    exp_not:N omnesia_action:nnnnnnnnn
     { exp_not:V l_omnesia_arg_i_tl    }
     { exp_not:V l_omnesia_arg_ii_tl   }
     { exp_not:V l_omnesia_arg_iii_tl  }
     { exp_not:V l_omnesia_arg_iv_tl   }
     { exp_not:V l_omnesia_arg_v_tl    }
     { exp_not:V l_omnesia_arg_vi_tl   }
     { exp_not:V l_omnesia_arg_vii_tl  }
     { exp_not:V l_omnesia_arg_viii_tl }
     { exp_not:V l_omnesia_arg_ix_tl   }
   }
  __omnesia_tmp:w
  group_end:
 }
keys_define:nn { omnesia / kvargcommand }
 {
  opt1 .tl_set:N = l_omnesia_arg_i_tl    ,
  opt2 .tl_set:N = l_omnesia_arg_ii_tl   ,
  opt3 .tl_set:N = l_omnesia_arg_iii_tl  ,
  opt4 .tl_set:N = l_omnesia_arg_iv_tl   ,
  opt5 .tl_set:N = l_omnesia_arg_v_tl    ,
  opt6 .tl_set:N = l_omnesia_arg_vi_tl   ,
  opt7 .tl_set:N = l_omnesia_arg_vii_tl  ,
  opt8 .tl_set:N = l_omnesia_arg_viii_tl ,
  opt9 .tl_set:N = l_omnesia_arg_ix_tl   ,
 }

ExplSyntaxOff

begin{document}

kvargcommand{
  opt1 = this is val1,
  opt4 = this is val4,
  opt7 = this is val7,
  opt6 = {this is val6, that contains a comma, so it has to be enclosed in braces},
}
{do something with one [#1] and four [#4] and seven [#7], the special case six [[#6]]
 and also some empty ones by default like (#2) and (#3)}

end{document}