On top of all the good advices here, I would say one important thing is to be able to fail quickly.
This is more on the engineering side, but if you can shorten the idea to client feedback cycle, then it's much easier to deal with all those requests.
It's also much easier to tell the business team "let's work on the what's necessary to get feedback on this feature (which probably mean not even an MVP) and we'll get result by the end of the month" than "I'm sorry, your feature has been deemed priority 12, we expect it to be complete in 6 months to 1 year, provided priorities do not change (they will)".
Once you get this feedback, then it's much easier to quantify things.
This is more on the engineering side, but if you can shorten the idea to client feedback cycle, then it's much easier to deal with all those requests.
It's also much easier to tell the business team "let's work on the what's necessary to get feedback on this feature (which probably mean not even an MVP) and we'll get result by the end of the month" than "I'm sorry, your feature has been deemed priority 12, we expect it to be complete in 6 months to 1 year, provided priorities do not change (they will)".
Once you get this feedback, then it's much easier to quantify things.