The problem with tying SS to a summoner spell is there is NO uniquely support summoner spell. Designing the jungle experience around the expectation that the Jungler will take Smite and then loading Smite up with a huge amount of utility that makes it rival other summoner spells works because you can then be reasonably sure that the Jungler will take Smite, just as you could have been reasonably sure that the Jungler will take Smite beforehand. Ie- it hasn't changed the presumption that the Jungler has Smite.
Most supports take either Heal + Flash or Exhaust+Flash but some do take Ignite+Flash. On that last one, why? and this is really important: well, because some ADCs take Heal+Flash in order to have safer laning.
Well, we can't tie SS to Flash because then anyone could/would take it (depending on the extent of the buffs). Can't tie it to Heal without potentially facing cases where the ADC takes it. Our best bet therefore is Exhaust, but as noted, there's no guarantee that supports will take Exhaust so it would just be more shoehorning supports into their box.
One option would be to create a support summoner spell but this has two problems 1) the CV/Exhaust-problem: we could tie SS to CV/Exhaust but we'd just be forcing supports to take CV... something they haven't done in ages... or mandating that they take Exhaust further reducing supports available options and 2) any sufficiently high utility Summoner Spell for supports would be tempting to other roles (just look at Heal and ADCs).
I think the "tied to summoner spell" route is fundamentally the wrong direction to take for supports.
It's arguable that Sightstone should just be removed from the game, as an outdated relic from the time when it was the support alone's duty to ward, and the vision trinkets should just be buffed instead.
The only healthy outcome I could see for giving Sightstone more upgrade paths would be something like letting Sightstone build from either Ruby or Tome and its final form having a meaningful passive (possibly as an enchantment). But the problem there is that it's difficult to make "lump of AP" work as well as "lump of HP" because a couple hundred HP go a lot further than 20-40 AP (especially with the utility scaling on supports)... (Lump of HP with an AP zit on the side... well...)
Oh, and for the love of God, Don't. Put. CDR. On. It.