It's better to use Bitcoin imho. All Fiat currencies are unstable, and it would be difficult to know which one to choose.
I don't know if you are being sarcastic or just trolling.. If you really think fiat isn't stable right now. I think you need a medical check up mate. Quote
It's not bad idea, but which exchange API is reliable for your idea and which exchange API is trust able for your idea
Well what's better than Google? It has an api for currency exchange rates. That can be used. Quote
Also, i'm sure few people will oppose this idea.
Duh! One doesn't have to be a Sherlock to figure out who these people will be. I think they will come to terms if the users push them to. 
I don't know whether the idea is viable but I do commend you for taking a creative approach, in search of a solution.
Thank you 
The fees are already linked to fiat in a way.
Lets remove that ambiguity and make it clear? Don't you agree with that? Quote
When many people consider that fees are too high they choose to delay sending a transaction or send it with a low fee and just wait longer for it to confirm.
Well this is the point where bitcoin becomes a tool for the elites and less of a payment system for the masses.Quote
As the price of Bitcoin increases the BTC value at which people choose not to pay will decrease.
Not sure about this part. Miners may decline to accept all the payments, what will happen then? They may argue that they want atleast 30Sat/byte etcs.. Have to think about this part..