Raise Max Wards
I usually support and I frequently end up being the only one on my team who wards, with maybe 1 or 2 others placing a ward with their normal trinket. I feel 3 wards max is low.
With a team total of 15 wards max, it seems like a lot of wards being placed - at almost every ganking/ or bush route someone might take. However, that's only when your entire team is warding, which is rare. I feel the ward max should be raised to 4-6 per person, even if it means slightly lowering duration/ raising cost.
If your entire team wards, that's 20-30 wards active at a given time - which would definitely cover any possible ganking/bush routes, but that is only in a perfect warding situation when your entire team is warding. To counter this, improve Revealing Trinket and other items that reveal wards so it is easier to clean up/ detect enemy wards.
Another suggestion that is odd and probably wouldn't work is that you could have a limit on wards based on the entire team. The whole team is limited to 20 wards total, no matter who's placing them it adds to this total.