It might sound far-fetched, but after reading Arcane Phoenix and Merwyrm sections in Monstrous Arcanum I have been thinking about chances they will show up in future HE army book.
The fluff and relationship tables at the end of the books shows two monsters being intimate to HE. The 8th ed. tendency to add a few monsters/buff-models to rare slots also seemed to suggest that their addition to HE 8th book might not be wholly unlikely.

To summarize what these monsters are(without writing down costs), Arcane Phoenix is more of a supporting model whereas Merwyrm is 'outright' monster, i.e., suited for charging headlong and smashing things apart.
The Phoenix's attacks are all flaming and the model itself cannot be wounded by flaming attacks. During CC, Phoenix can 'burn' models in base contact. It also gives penalties to non-magical attacks when such attacks roll To Wound aginst the phoenix. And when killed in CC, it has 1/3 chance of reviving with few wounds. There are several upgrades including providing re-rolls for Ld tests, getting magical attack or ASF, and most shocking of all, providing Frenzy to stoic and abstinent HE near the Phoenix. While its offensive capability is slightly better than that of a Great Eagle, it's still tough and has decent number of wounds. Add aforementioned resistance to non-magical attacks, revival special rule and HE have an excellent tarpit model for dealing with skavenslaves, gobbos, gnoblars, etc.

Merwyrm on the other hand have stats similar to that of a 'normal' dragon(WS6 S6 T6, 3+ Scaly Skin). It is so far the only model with Sea Creature Special rule and can regenerate, making it one tough monster. Its attacks are poisoned and can boost up the strength of one of its attacks, which is strong enough to wound most things on 2+. Even TK sphinxes aren't wholly safe from this single attack.

So far, both models can be excellent additions to HE, fluff-wise and game-wise. Phoenix can provide good buffs to nearby units while serving as good tarpit for large block of S3/4 troops, while Mermyrm is more-than-decent monster with good durability. However considering that new 8th ed. books include at least one box set which can be assembled in two types of models, I am not sure whether these two monster can be an adequate candidates for the new HE book.(If they can get one)