I think it would be interesting to have a new subscription, cheaper, and that does not need to have the games in 4k, 1080 is already cool for most people!
But it needs to have enough games! it would be the same Stadia PRO games, but it needs to have at least 100 games both subscriptions (some people are more interested in quantity than quality)
It would be good in countries where the dollar is more expensive like in Asia and Latin America, and it would require less bandwidth in these countries!
And it could be called Stadia Plus
Or even Stadia PRO with the current Stadia PRO being renamed to Stadia PRO 4K
$6 would be a nice amount!!
Great ideas @Henrique
You should make sure to let the Stadia team know your suggestions by using the feedback feature built into the app. If you click on your profile icon, you should be able to find the area to leave feedback. Best of luck!