Illumination Flare
-Reveals INVISIBLE enemies who linger in the area.
Seeing that we have a new sneaky guy in the hero pool, we need to start having counters against them. This won’t make a dramatic impact but its a start.
This is exactly my line of thinking as well. And also maybe the marks by Nightingale, Prophet should do the same.
We need anti-invis.
Yes! 
Totally agree with Night and Prophet’s skills to reveal invis. Might be a stretch, but i think silence should also reveal invis. Cast’s sea mine to be more specific.
Even though there are existing counters that can bypass invis (Oro’s meltdown, Razorback’s shield when it pops, Fortress’s recharge and ground lingering skills and traps), I still think invisibility is too strong in this game since it basically makes the AI useless against them. (except sentry’s, I think that skill should be charged faster or lasts longer than 6 seconds to compensate for his squishyness)
And as much as I love and hate mandrake, I dont want him to be nerfed. Just keep him as ease. Instead, I suggest introducing new ways to counter invis.
Ghoul’s molotov could also disable the invis too