The passive wouldn't be good as you'd have people spamming wards at fountain every back once they got sightstone and/or would pay way too little per ward in early game and/or have too long a cd to be on par with the other support items.
Most champions these days (including "AD" ones) have some ap scalings in their kit somewhere, so the spelltheif line is usually a good option for pokey AD's, and then they can try to build AD beyond that. CDR is good on almost everyone except those who rely almost exclusively on AA's so the coin line is a reasonable option for many AD 'supports' too. AD bruiser sorts like having health (you even suggested the ad support item have health!), so the relic line isn't useless either.
The thing is AD scaling champions are some of the most gold reliant instead of level reliant, so they're almost certainly going to fall behind on a supports budget even if there was an AD support item tailored to them. If there was an AD support item that provided comparable gold to the adc (his/her)self, then the ADC would be taking it (and indeed, probably most champs would, as getting gold comparable to a lanes worth of cs without needing to cs is super OP). If for some reason you want to be a relatively useless mini-adc, then you can be with the current item pool, but it's important to avoid 'noob trap' items that would tempt newer/lower elo players into trying it. It can be fun for experienced people to screw around with, particularly when they're confident they'll snowball, but being a gold starved adc is pretty much objectively worse than alternatives.
If the goal as an 'ad support' is actually to all in and snowball off kill gold, then you'd want to start with dorans blade or a long sword anyway, since an AD sup item would almost certainly give less ad, and you'd want to maximise ad for the all in. If you're simply someone who has AD scalings being chosen for the utility (knock ups, etc), then there is a very strong chance you're better off being built tank, since the utility itself doesn't scale with AD in the majority of cases (it's a similar reason to why even most 'ap' supports typically shouldn't build AP, as utility items like locket tend to be better. Some AP supports do have their utility scale with AP to be fair though).