I have used busybox init for a while. It differs from sysvinit
by not having runlevels, except perhaps on and off.

Busybox repo is at: git://busybox.net/busybox.git
Website at: https://busybox.net/
Comment re. systemd: https://busybox.net/kill_it_with_fire.txt

Example inittab at:
https://git.busybox.net/busybox/tree/examples/inittab
excerpt:

# Note: BusyBox init works just fine without an inittab. If no inittab is
# found, it has the following default behavior:
#       ::sysinit:/etc/init.d/rcS
#       ::askfirst:/bin/sh
#       ::ctrlaltdel:/sbin/reboot
#       ::shutdown:/sbin/swapoff -a
#       ::shutdown:/bin/umount -a -r
#       ::restart:/sbin/init
#       tty2::askfirst:/bin/sh
#       tty3::askfirst:/bin/sh
#       tty4::askfirst:/bin/sh

The sysinit thing is a how you start the system, it's just
one script /etc/init.d/rcS (or where you prefer to place it).

One could possible (not tested) mimic sysvinit bootup by having
for i in /etc/rc2.d/S*; do $i start; done
in the rcS file or if you prefer you can have your own handwritten
and tuned script there.

Would there be any interests having a busybox_init package which
I could possible maintain (with a little help) ?

Regards,
/Karl Hammar