With bitcoin dropping to below $8k, total market cap also dipped below $400B. I think your prediction to stay within $300B and $600B is probable. It would depend on how quickly bitcoin and altcoins can retrace their previous ATHs. I think by June, we'll be back to $700B if not higher as several big ticket projects are set to launch at this time..e.g. EOS project of Lanimer; YOYOY ecosystem which is Steemit's Chinese equivalent, IOT etc.. 2018 will for sure be an exciting year for blockchain but as to which coins will dominate the year, very hard to tell at this point.