Short answer?
Scrap the damage bonus, give it baseline haste and keep the haste per stack low. Also scrap LI, Chorus, Mass Hysteria, and any other thing that gives VF power. VF should be haste, bolt access, and maybe mind blast cooldown reduction.
Drastically reduce time taken to get into VF. Instant eruption is something people want but I could take or leave, the none lotv cast time is horrid though.
Remove cooldowns that extend voidform beyond base length baseline, longer VF is a playstyle choice, and talents that offer gain in the immediate should be a viable alternative for where that playstyle wouldn’t be optimal.
Tweak base drain down, but drain per stack up, so that shadow has a realistic stack cap (20-25 or so) that doesn’t go up much at all as people gain gear.
Remove AS, and dont add any way to gain insanity from other stats. Haste should be the only stat that increases insanity gen, so gear doesnt scale with it too well.
PUT COOL THINGS AT THE START OF VOIDFORM SO PEOPLE WANT TO BE IN IT, INSTEAD OF FEELING LIKE THEY NEED TO GET TONNES OF STACKS AND AREN’T SAD IF THEY DROP OUT EARLIER THAN EXPECTED.
NH 4set, and Legion Beta Sphere of Insanity (It never went live so if you dont know what it was it might be hard to find) are great examples of how to do that, which already have existed.
Rework basically every talent, but at least SWV, LI, AS, S2M, And the entire last row.
Yes this list is enormous, and it’s also just my opinion, but the point I want to make is shadow needs an absolute metric tonne of work no matter what playstyle you enjoy. Since blizzard took reworking shadows resource mechanic off the table, this is a laundry list of everything they by their own admission are still capable of doing.