Seeing as I have a history with a more harsh government style, you won't be surprised to find that I believe the cap for those not endorsing the delegate should be 1. If they are draining influence from the region, they should not be rewarded. A nation needs two endorsements to make WA proposals. If they want to live a quiet life of WA participation, they can pay the tax of their endorsement. Otherwise, I think Arch's tiers are pretty good.
Keep in mind this is going to link the forum community activity levels back into the in-game endorsement levels. It might take some fancy legislating and be a bit of a hard sell.