We will never, ever see a klingon with a flat forehead again. Only in case of a joke, like the Tribble episode on DS9. The Enterprise two-parter was an anomaly, where they knew the series was ending, and were allowed to go wild. And in my opinion it was still a mistake to adress the make-up change on-screen.
But don't expect to ever see a TNG-klingon again, either. I highly doubt anyone will take a space-viking race serious anymore. If we see klingons again (and we certainly will), expect them to be re-designed again, maybe closest to the "Into Darkness" klingons in appereance. The look informed by their past appereances, but completely overhauled anyway.
The Undiscovered Country klingons are my absolute favourite portrayal of them ever. Because you could take them seriously. The mix of their warrior code with a strong militaristic organisation made them seem effective. They weren't so much talking about "honour", but instead acting on their (by a military leadership distorted) sense of honour.
I guess if we will ever see klingons again, having them redefined by Nicholas Meyer is one of the best things being able to happen to us Trekkies.