­Rainbow Vale is pretty much perfect as it is.

Make it change contoller immediately, to avoid memory issues, though. It does prevent repeat untaps; but this set doesn't really have any of that so I'm not particularly worried.

Nor, really, is this a set which demands this card. It's got loads of colour fixing for no particularly good reason. Eh, whatever. It's a classic.

(Note to self - duh. Legends was around with this set; which had lots of creatures with stupidly high multi-color costs. So this set probably contained some attempt to make them castable. For both players who owned more than one booster pack of legends because boy did they underprint that set and overprint this one.)

The likely reason this changes controller at end of turn is that mana abilities fully resolve during casting of a spell and if the change of controller is followed by other effects there is a risk of rules issues.

A perfectly safe wording would use a reflexive triggered ability that only resolves once you are done casting (so no actions can be rolled back anymore), but otherwise with as little delay as possible:

• "An opponent of your choice gains control of Rainbow Vale." >> "When you do, an opponent of your choice gains control of Rainbow Vale."