I do like this thread, because it allows us to look back in the future and see all these 'ridiculous' predictions (that's how they will probably appear in retrospect). Moreover I think it i impossible to predict the value of Bitcoin in 20 years. If Bitcoin succeeded it will be very high, since a lot of people will use it as a store of value (these coins won't be traded) and the small percentage that is traded has to be divided over millions of hands.
2020 is a more interesting timespan at the moment. From 1995 to 2000, the internet captured from 0.4 to 5% of the world population as users. We all agree that Bitcoin will be able to do this quicker, since it is actually something that is built on top of the internet. So let's say 10% of the world population will use Bitcoin by 2020. That is already 700 million people. This could be done if the userbase tripples more or less every year. Not completely unrealistic.
In between this timespan there are two blockhalvings. At the end of 2020, only 6.25BTC per block will be rewarded. This means that also less 'freshly mined' coins will enter the market. Most of the supply will come from long-term holders or traders.
Last but not least you should ask yourself, will there be much incentive in 5 years to exchange your Bitcoin for fiat currency. If you can just use your Bitcoin almost everywhere and it gives certain advantages such as lower fees, less risks and so on, why the hell would you sell your Bitcoin in that case? In the end, merchant adoption and Bitcoin integration in complete business infrastructures will take away some of the pressure to sell your Bitcoin, which is positive for the exchange rate with fiat.
Imagine if you could pay everywhere with (digital) gold. Would you still change it for fiat?
To make a long story short, I am pretty bullish on the long term. The two halvings, the merchant adoption and Bitcoin integration in business as well as the growing user base will have a positive effect on the exchange rate. Exchange rate around 2020 can therefore easily be between 10.000 and 50.000.
If you would say that you need in most Western countries 30 year salaries to start living on one's private means, one million USD would be a nice goal. With the minimum of 10.000 by 2020, you would thus need 100 coins. Chances are that 50, 30 or even 20 coins will suffice however. And if you decide to retire in Indonesia or Thailand, you could probably live with much less, say 5 to 10 Bitcoin. So anything from 5 up to 100 Bitcoin gives you a reasonable chance in my eyes to reach your goal.