Interested to hear what you class as third party development? Software, Hardware, Services, additional core development?
Mycelium integration,
closed source pools with their own closed source software, previous releases of
cudaminer not being released for distribution and are now effectively closed source, coinmarketcap, any exchange, there are now third party sources of entire
coin development who offer the source for anyone to use but not for themselves to launch a coin,
claymore's GPU miners are closed source 3rd party support, nxt asset exchange,
blockchain explorers not provided by the developers supporting multiple coins of choice,
profitability calculators, there's just so many examples. The future of new cryptocurrencies will come to thrive on these types of offerings. This is no longer a garage project, and the number of people who have originally provided these types of services for free of charge is quickly diminishing .. to the point where coins like BBR are
left to consider a hard fork because of lack of an ability to fund a third party developer.
Open from the start, multiple contributors, patches from healthy up to date forks - the usual methods.
This simplistic model is disappearing very fast, again using the
BBR example. He is a very intelligent and good developer and even he cannot mitigate these risks.
The fee model is only to provide a fair approach to remuneration. All aspects of the project would have to be open, right from the start, including the initial requirements and proposal, even before specification happens. As we are doing here

The fair approach is being actively abused, and I'm trying to drive home the point that it's not just the core coin developers that will be needing paid anymore to survive. It's quickly becoming so dangerous that these people need to be compensated some how.
So what do you think is a good method to keep everyone paid? I don't think it's going to be automatic -- no amount of automated open-source coding can benefit everyone who has a say in these coins now. Some will find a way to assume control if they are not compensated.