Fix sentry wards or remove them completely (Players have enough options to reveal invisible units already)
Slightly nerf the invisible units : They should not be more deadly than other units spawning at that time on top of being invisible. I'd suggest to reduce their hp by 50%. It would still punishe players for not getting ways to detect them/kill them quickly.
Disable medic auto cast heal when they spawn : since healing wave cost their whole mana pool, they often are unable to cast it right away since they'll spend some mana on a wounded unit close by.
Have heroes deal the same attack type - Hero attack. It would help make some heroes more useful. For example the ranger barely sees any play because piercing damage is terrible against bosses. On the other hand we have the alchemist (supposedly a support hero) which has critical strike and hero attack and therefore is way more effective than the ranger (both at killing bosses and supporting the player's economy).
Have the early bosses (like the first 3) have a timed life otherwise they can be used as meatshield until the very end
Remove the normal factory from the tinker build menu and set the upgraded factory's hotkey to F (fixes some hotkeys and there is no reason to get the normal factory with the tinker)
Change exterminator's attack to normal (like the AA towers). Right now they are too expensive for their damage output.
Oh and thanks for this awesome game Nevo