I'm currently playing an asura-spawn tiefling born from two elven parents who features elongated ears and (for thematic reasons) I had it start with 12 constitution as a result.
For traits, you could take adopted to gain one racial trait, but if you wanted to houserule then I would consider the following.
Tieflings do not have access to their parent race's favored class options, archetypes, feats and so on. if you take the pass for human alternate racial trait then you bypass this specifically for humans. Consider asking your DM to allow you to choose this trait and key the subtype to the race you wish to mimic. For example, replacing humanoid (human) with humanoid (elf) and calling it 'pass for elven'.
This isn't what I would call a game-breaking houserule. However, I would say that choosing a hybrid race with it (such as half-elf) would only let you count as half-elf for options (not human, elf, and half-elf; just half-elf).
There is one thing of note: rules exist for small tieflings. You can choose to make them small. This doesn't change their stats. What changes occur are just size bonuses and penalties to skills and certain rolls and defenses, such as +1 attack, +4 hide, -25% carry capacity, or -1 CMD. Weapon sizes carried default to small, too.
The reason this doesn't change your ability scores is because size changes for PCs don't currently have default adjustments. All adjustments have specific changes based on effects used.
A good houserule would be to allow this for skinwalkers. There is precedent, and it makes sense.
On a final note, a race which can select pass for human can legally use it to choose the feat racial heritage. In this way a tiefling can spend a feat to gain access to human, tiefling, and a third race's options. Not bad.
It does lock you out of some of the fun tiefling racial options but it's a nice combo. Certain other races such as aasimar have this option.
Apart from that, there's no real way I can think of to have a mechanical change when you're of a different humanoid parent without third party options or house rules... for good reason. Why even be a kobold with its tragic stats if you could just be a tiefling kobold and pick all the good options, right?