I think 600 billion by end of year is possible, but I see this as a best case scenario. If you look at your chart market cap growth has stalled for some time after doubling. I think growth will start again but we may have to wait for the market to fully digest the last increase in market cap.
The market needs to recover from this crazy ICO phase we are witnessing. I feel like the bancor and status ICO definitely put unnecessary strains on the ETH network which ultimately hurt the whole crypto market.
http://www.coindesk.com/ico-blues-status-raises-58-million-leaves-buyers-waiting/