For me, some of the things that's really important for a terminal multiplexer include:

1) Keybindings having no conflicts with other terminal applications. For that, vim's default window-management key of <C-w> is unfortunate, as it is very important for delete words in bash and readline. I remap that to <C-@> (CTRL-Space) instead, which is basically used by no terminal program that I am aware of. The other key binding that trips me up are the default vim keybindings to kill the terminal (<C-w><C-c>). If I am midway attempting a window action, but realise something is taking too long and want to stop the job, the keybinding kills my whole shell. If I really want to kill my terminal, I can always issue ZQ or :q! in normal mode.

2) Change my cursor shapes based on which mode I'm in (normal, insert, or terminal/command), which helps me easily tell the mode.

3) Improve the default keybinding for going from terminal back to terminal normal mode. I find the default keybindings <C-\><C-n> and <C-w>N too difficult to type because I need to use this binding very much.

4) Be able to move between windows with the same, consistent set of keybindings no matter which mode I'm in (e.g. not having to go back to normal mode first, from insert or terminal mode). In the same vein, when I land on a buffer I always expect to be in normal mode, so I can start moving around immediately and yank stuff without remembering whether I'm in a terminal or a non-terminal buffer. When I need to manipulate the terminal program I can always `i` or `a`.

I have quite a few other customisations to my vim terminal, but I think these are the most essential ones.

vimrc for 1)

set termwinkey=<C-@>
noremap <C-@> <C-w>
noremap <C-@><CR> <C-w><CR>
noremap! <C-@> <Esc><C-w>

noremap <C-@>c <Nop>
noremap <C-@><C-c> <Nop>
noremap <C-@><C-q> <Nop>
noremap! <C-@>c <Nop>
noremap! <C-@><C-c> <Nop>
noremap! <C-@><C-q> <Nop>
tnoremap <C-@>c <Nop>
tnoremap <C-@><C-c> <Nop>
tnoremap <C-@><C-q> <Nop>
noremap <C-@>q <Nop>
noremap! <C-@>q <Nop>
tnoremap <C-@>q <Nop>

vimrc for 2) and 3)

augroup term_normal
au!
autocmd WinNew * let w:winnew = 1
autocmd CmdlineEnter * let w:outcmd = 0
autocmd CmdlineLeave * let w:outcmd = 1
autocmd WinEnter * if exists ('w:winnew') | unlet w:winnew | if (mode() == 't') | call Termcursor () | endif | elseif get(w:, 'outcmd', 1) && (mode() == 't') | call feedkeys("\<C-@>N", "") | endif
autocmd WinLeave * if (mode() == 't') && (! exists('w:tu')) | call feedkeys("\<C-@>N", 'x') | call Normalcursor () | elseif (mode(1) == 'nt') && (exists('w:tu')) | call feedkeys("i", 'ix') | endif

tmap <expr> <C-@><C-@> ((mode() == 't') && get(w:, 'outcmd', 1)) ? '<C-@>N' : '<C-@><C-@>'
augroup end

let &t_SI = "\e[6 q\e]12;#FF00DF\<C-g>"
let &t_EI = "\e[2 q\e]12;#FF00DF\<C-g>"
let &t_ti = "\e[4 q\e]12;#FF00DF\<C-g>"
let &t_te = "\e[2 q\e]12;#FF00DF\<C-g>"
let &t_Us = "\e[4:2m"
let &t_ds = "\e[4:4m"
let &t_Ds = "\e[58:5:4m\e[4:5m"

function! Normalcursor ()
let &t_ve ="\e[2 q\e]12;#FF00DF\<C-g>"
let &t_vi ="\e[2 q\e]12;#FF00DF\<C-g>"
endfunction
function! Echonormalcursor ()
call echoraw("\e[2 q\e]12;#FF00DF\<C-g>")
endfunction
function! Termcursor ()
let &t_ve ="\e[4 q\e]12;#FF00DF\<C-g>"
let &t_vi ="\e[4 q\e]12;#FF00DF\<C-g>"
endfunction
function! Echotermcursor ()
call echoraw("\e[4 q\e]12;#FF00DF\<C-g>")
endfunction

augroup termcursor
au!
autocmd ModeChanged *:n* let &t_ve ="\e[2 q\e]12;#FF00DF\<C-g>"
autocmd ModeChanged *:n* let &t_vi ="\e[2 q\e]12;#FF00DF\<C-g>"
autocmd ModeChanged i:* if &t_ve == "" | let &t_ve ="\e[2 q\e]12;#FF00DF\<C-g>" | endif
autocmd ModeChanged i:* if &t_vi == "" | let &t_vi ="\e[2 q\e]12;#FF00DF\<C-g>" | endif
autocmd ModeChanged *:i* let &t_ve =""
autocmd ModeChanged *:i* let &t_vi =""
autocmd ModeChanged *:t call echoraw("\e[4 q\e]12;#FF00DF\<C-g>")
autocmd ModeChanged *:t let &t_ve ="\e[4 q\e]12;#FF00DF\<C-g>"
autocmd ModeChanged *:t let &t_vi ="\e[4 q\e]12;#FF00DF\<C-g>"
autocmd ModeChanged t:* call echoraw("\e[2 q\e]12;#FF00DF\<C-g>")
autocmd ModeChanged t:* if (mode(1) != 'ct') | let &t_ve ="\e[2 q\e]12;#FF00DF\<C-g>" | endif
autocmd ModeChanged t:* if (mode(1) != 'ct') | let &t_vi ="\e[2 q\e]12;#FF00DF\<C-g>" | endif

let g:cmdlinedepth = 0
autocmd CmdlineEnter * let g:cmdlinedepth = g:cmdlinedepth + 1 | let &t_ve = "\e[4 q\e]12;#FF00DF\<C-g>" | let &t_vi = "\e[4 q\e]12;#FF00DF\<C-g>"
autocmd CmdlineLeave * let g:cmdlinedepth = g:cmdlinedepth - 1 | if g:cmdlinedepth == 0 | if (mode(1) == 'ct') | call Termcursor () | else | call Normalcursor () | endif | endif
autocmd CmdwinEnter * let &t_ve ="\e[2 q\e]12;#FF00DF\<C-g>" | let &t_vi ="\e[2 q\e]12;#FF00DF\<C-g>"
autocmd CmdwinLeave * let &t_ve = "\e[4 q\e]12;#FF00DF\<C-g>" | let &t_vi = "\e[4 q\e]12;#FF00DF\<C-g>"

" autocmd WinEnter * call echoraw("\e[2 q\e]12;#FF00DF\<C-g>")