The main challenge for Bitcoin in January is not to give way to the bears who are betting it would tank. It should resume to rise, instead, reversing this short time bearish trend.

What problems can be in bitcoin? The main problem will be if he very much loses in value, which is unlikely

Bitcoin haven't hit a high price for this month as compare to what we have experienced last nov. And dec of 2017. Maybe this isn't a good.month for bitcoin considering also the high charges. But let's not lose hope and stay optimistic this is just the start of the year, bitcoin have a long way to go. Surely it will recover from all of this.

in this january i think the market is very bullish and it can go to any of the two altcoins and king bitcoin if bitcoin rallies again and gain momentum alts may go with bitcoin or it may go down depending on the trend of news and fuds that is happening, many challenge is ahead and i think there will be more fud this time and a lot of threat against bitcoin like forking and a lot of disagreement about bitcoin

For 2018 the bitcoin can get to Bend your price and reach 40k $ but it can also suffer fall pronounced reaching less than 10k $ having a great volatility

Some experts say that it is a bubble waiting to burst because its supply and demand is not equal. There is a huge demand for it and it lacks money to supply it. Another thing is the rise of other cryptocurrencies like Rippler.

At the last days of December 2017, we experience continuous deflation of value of bitcoin. The said deflation of bitcoin value might associated with the holiday season.
 Unfortunately, many investors who bought bitcoins while its value increases above $17,000 experience a great lost. This deflation of value might give impact on the first quarter of 2018.  Those people who experienced great lost might loose hope and confidence in bitcoin and winning back their trust is a challenge.  The good thing about this is during the first week of January, the bitcoin value start to recover.  Will we meet the goal of $25,000? Of course it is possible. If we trace the bitcoin milestone last year, the value between the first quarter compared to the bitcoin value on its last quarter shows a great difference.  The value increases for about 700%. If the bitcoin community will grow this year (2018), it is also possible to increase its value 700%.  Meaning, it will hit even more than $25,000.00

I think we are experiencing the challenge in bitcoin right now, we felt the high price and slow transaction process. Investors are little afraid about this situation and it may affect the bitcoins economy.

there are always lots of challenges ahead of bitcoin. but since you are focusing on the price so i will tell you the main price-related-challenge!
it is the weak hands that keep coming in bitcoin. whenever bitcoin price rises, which has been every single months lately, people see it as a good investment and mostly as a "get rich quick" thing. and so many of them are idiots with money at hand who want to become rich just because they bought bitcoin at some arbitrary price and whenever there is the smallest FUD saying the words "bubble", "Fraud", "crash",... they start thinking all their dreams are shattered and start dumping causing havoc on the market where only whales benefit.