I would still like Constitution changes due to polymorph to affect HP...and IMHO it would still allow a perfectly legitimate tactic of turning yourself into a type of creature that would increase your chances of surviving a longer battle or series of.
My approach has been entirely different. For my D&D 3.25e game, I generalized the notion of a hit points from size class bonus seen in RAW in such things as oozes and constructs. All creatures now recieve bonus hitpoints based on size class. This has had several very positive benefits on my game. First, it addresses the classic 'house cat problem' adequately, in that a house cat no longer is clearly more dangerous that his farmer owner. Second, it addresses the 'whale' problem that D&D has had since the beginning, allowing me to have large creatures in the world with lots of hit points without relying on giving them enormous hit die and all that comes with it. I can give a whale 5-8HD and it still have enough hit points to feel like a whale, while not having a +20 BAB, enormous numbers of skill points, and other sundry undesirables. Lastly, it does pick up something from 4e that I did like, which was that you know longer have to treat 1st level as a special case because of the extreme fragility of a 1HD PC. There is also a more gentle transition between 1st and 2nd level.
In my game, one of my players 'abused' my system (in precisely the way it was intended to be abused but in a way I'd never expected) dipping for 1 level of the custom 'Feyborne' class (which lets you gain spell-like abilities) to build what is most easily described as an alternative Barbarian. Instead of going 'Beserk', he could turn 3 times per day into a 12' tall (blue skinned) giant. This proved to be a lot of fun, and was basically balanced with his prior Beserker build (after that character died). The giant basically gave him most of the benefits of going beserk - increased damage and temporary hit points - plus greater reach, but cost him an action compared to the free action of going beserk.
That shows I think my general approach to the problem. Define what you can shape change into as a buff. That buff can be combined with some sort of appearance (in the case of something like Polymorph, a very varied appearance) but mechanically what you are dealing with is simply a customizable buff. Instead of trying to limit what monster you can change into, simply limit the buff.
If I see 5e going that direction, I'll know that the designers have a handle on things. If not, well, I haven't lost anything. The SRD gives me ownership of the game. They can take away my game, but they can't take away my freedom.
