If you want to fix him without remake, you must buf his abilites by 30% not less. (Please don't tell me i don't know how to play him.)
I like to play mid laners and my favorite ones are: brand, lux, ziggs and malz before rework.
Look at abilities for example:
1) Brand has 4 damage skills + CC (stun) + extra damage blaze + extra damage "3 hits from skills". Easy to use ult. (mostly AOE damage)
2) Ziggs has 4 damage skills + CC (clow) + CC (push) + extra damage from passive + extra damage to buildings. Easy to use ult. (mostly AOE damage)
3) Lux 3 damage abilities + CC (clow) + CC (root) + AOE shield + extra damage passive. Easy to use ult. (mostly AOE damage)
I can write similar list for other mid laners not my favorites but most of them are heavy damage dealers + CC or support abilities or mobility (assassins).
What with the malz?
3 damage skills + CC (hard to use ult, usable 1v1).
With his low range slow attack (no burst), long cooldowns he can't stay in team fights, he must place skills and run away doing nothing. Before rework i could place W (field), E and sometimes help kill with Q or autoattacks. I could zone enemies with W, i could use it as wave clear when enemies siedge my tower. Now i can't do anything. His Q has not enough damage and range, hard to use properly, and E needs long time to bring damage. His passive (oh don't tell me you don't know how to play around it) is useless actually, because on cooldown when you really need it. Current W is just useless, people ignore voidlings in teamfights.
To make malz good again you must replace this useless VOIDLING (W) for something useful!
1) Add 4th damage ability like old (W) field usable in team fights and help wave clear.
2) Support / CC ability: shield on demand with short range teleport to help him survive or mass CC/shield to help team.
P.S. I hope i will play him again some day.