The real way to fix sightstone
First off I'd like to state that I don't think that the current 
has any problems, aside from not having enough wards stock. That being said the upgrade for it utterly useless. 
is almost solely bought as the last item in a build. It gives a slight more bit of health and increases the stock by 1. It has no appeal as a mid game purchase. I have a theory on how to actually fix this issue.
First off needs to generate wards over time OR have an increase in how many it holds. That is the only change that the sightstone needs.
There should then be a few different upgrade paths. The point of having upgrade paths isn't to give stats though. The point is to increase utility of warding and as such the upgrade does not increase stats. Keep in mind that some of these may be a little outlandish and/or unneeded but sounded good in concept. Similar to boots upgrades and jungler upgrades these would be non-stackable.
Note: all upgrades are exactly identical to with the exception listed in the description.
First upgrade option (Sightstone Sweeper) When used to place a ward, the ward gives true sight for ~2 seconds then acts as a normal ward.
Second upgrade option (Sightstone Trap) Wards placed will snare the first enemy to walk over them for ~1 second with a ~2 second arm time.
Third upgrade option (Sightstone Scanner) Wards will scan the area once every ~20 seconds and reveal stealthed units (not traps or wards) for ~.5 seconds
Fourth upgrade option (Sightstone Revealer) Wards will create vision over walls within their reveal zone.
Fifth upgrade option (Sightstone Ranger) Wards will "walk" around the area they were placed in within a 500 unit radius. They walk to the edge of the radius or to a wall then turn at a random angle and walk to the edge of the radius or a wall and repeat the process until they die.
Sixth upgrade option (Sightstone Oscelot) Periodically the ward will spawn a drone that reveals an area within ~700 units (~200 unit radius) and moves ~500 units over ~1 second. The spawn has ~15 second cooldown. (The thought process here was the oscelot minions from the bilgewater event)
I have more, but these seemed like a good start of ideas to throw out.
