Speaking from the MultiBit perspective, all future protocol development (with the exception of critical security and network compatibility fixes) will be put into a HD wallet. Over time we want to see "MultiBit Classic" gracefully retire and be fully superseded.
Right now, HD is not out there but there is a lot of work going on between wallet developers to harmonise on HD implementation through BIP32/39. The result of that work should see a significant migration away from random private keys.
Thus it would appear likely that by the time this protocol sees widespread use the presence of HD is likely to be rising fast or possibly dominant.
At MultiBit we anticipate a release of HD code within 2 months, with private beta occurring within weeks. Trezor and Electrum may be earlier than this. As far as I am aware both Hive and Haskoin are committed to HD.
If anyone wants early access to the alpha code, let me know and I'll make the arrangements.