Few words could summarise the year we witnessed in 2020, with investors taken on a roller coaster of emotions. Financial markets were beneficiaries of an unprecedented level of fiscal support, while a record number of new investors signed up to invest in the stock market for the first time.

Off the back of last year volatility, many observers expect a more subdued year ahead for the stock market, however, there are still a host of considerations investors should think about if investing in shares in 2021.

How soon will the vaccine rollout open the global economy?

By far the most pressing issue this year will be the progress of vaccine rollouts across the world. Aside from the public safety element, this is seen as a lynchpin to reopen the global economy. It will be a task full of challenges, including manufacturing and logistical bottlenecks, let alone the unknown timeline as far as reaching herd immunity.

Thus far, prior to vaccines, pent-up demand has driven retail sales growth. However, much of this has been spurred on by government stimulus, and as seen in the US, once this stops, growth fades. As such, a vaccine is all the more important to drive consumer confidence and business sentiment.

International travel and border reopenings will also be pivotal for the economy. While the likes of Qantas (ASX: QAN) are optimistic that international travel could start from the middle of this year, government and medical officials have suggested that the entirety of 2021 may even prove unlikely given the scale of vaccine rollouts required not just here, but in other parts of the world. Even the prospect of travel bubbles remains unclear given the response to local COVID outbreaks recently.

With all the uncertainty, travel, leisure and international education remain some of the vulnerable sectors in 2021, likely to continue dividing investors' and drawing notable trading interest.

How might the stance of global central banks change?

Last year, central banks around the world embarked on an unprecedented effort to stimulate their respective economies. Whether it was interest rate cuts, quantitative easing or various other measures, no stone was left unturned in an effort to help bring stability to the global economy.

These efforts acted as a catalyst for financial markets. With that in mind, the US Federal Reserve Board meetings will be watched closely throughout the year, including its trajectory for easing bond-buying activity and increasing interest rates. The bank most-recent forecast suggested this would be years away and accompanied by advanced notice, however, one central bank official recently suggested that next year might be in play.Closer to home, the Reserve Bank of Australia is tipped to maintain interest rates at rock bottom levels potentially until 2024. But with the Australian economy faring significantly better-than-expected, the country relative success in quashing COVID, and a rising Aussie dollar, any shift in this timeline runs the risk of catching the market off guard.

Will trade tension between the US, China and Australia subside?

With the change of government administration in the US, it remains to be seen whether there is a political reset as far as the relationship between the United States and China.

While it is possible the hostilities between the nations will tone down, investors will be looking to see whether geopolitical policy initiatives from the Trump administration are wound back. This might not only prove easier said than done, but it may also fall down the pecking order as the Biden administration focuses on its domestic agenda and containing COVID.

Arguably more important for the ASX, however, will be Australia relationship with China, which deteriorated significantly in the second half of last year.

With industries from beef to wine to barley all coming under attack, the economic fallout has been gradually building, with certain companies such as Treasury Wine Estates (ASX: TWE) hit hardest. In the meantime, China appears to have been the only major economy to grow in 2020.

Investors should monitor which direction the Sino-Australian relationship moves in 2021, however, in the meantime, it may mean that certain stocks heavily dependent on the Chinese market and bilateral relations could continue to see subdued sentiment.

Is the commodities boom set to continue?

Just over a year ago, in our ASX market outlook piece for 2020, we posed the hypothetical question as to whether the price of iron ore had peaked. The resounding answer to that question was no, as evidenced by the commodity recently passing US$170 per tonne.

It has spurred on the likes of BHP (ASX: BHP), Rio Tinto (ASX: RIO) and Fortescue Metals Group (ASX: FMG) to all-time highs. With the supply of iron ore out of Brazil still severely disrupted, and China thirst for the commodity showing no signs of abating, the cycle is certainly more pronounced than anyone expected and will come under consideration in the year ahead.Meanwhile, gold had a stellar year in 2020, gaining 22% in USD terms. The move sparked strong performances from companies such as Saracen Mineral Holdings (ASX: SAR) and Evolution Mining (ASX: EVN), even though that rally faded in the final quarter of the year. Now, with the prospect of more government spending and stimulus in the US, and some concerns around inflation, gold is sure to remain a hot topic in 2021.

Elsewhere, copper is trading at its highest level since 2013, while lithium stocks have caught a surge of interest in recent months as electric vehicle development heats up, and as the price moves away from its historical bottom.

Oil has started the year around an eleven-month high, something that would have seemed unlikely when it dipped into negative territory last year. With LNG prices also hitting record highs, the commodities cycle has started the year in full swing. Whether this fades is something investors will need to be on guard for.

Where will the Aussie dollar move in 2021?

Since the depths of the market crash in March 2020, the Australian dollar has steadily risen from US 55 cents to as high as US 78 cents. There have been several factors contributing to this trend, with the pattern thus far not too dissimilar from that seen during the Global Financial Crisis.

One of the biggest drivers has been a weaker US dollar on account of monetary printing and major government spending in the United States. Furthermore, a risk-on attitude towards assets, something that typically works in favour of the Aussie dollar, has also played a part.

Australia ability to quash the coronavirus has also provided optimism to global currency traders around the prospect of a better economic recovery here than abroad. And finally, the strength of the commodities market, particularly iron ore, has also given rise to a stronger Aussie dollar thanks to the influence that this sector has in driving Australia domestic GDP.

A stronger Aussie dollar, however, is in itself seen as a headwind in some quarters for economic growth, especially as major exporters that earn most of their earnings or report in USD benefit from a weaker Australian dollar.With a higher AUD/USD rate, some of the stocks affected include CSL (ASX: CSL), Brambles (ASX: BXB), Amcor (ASX: AMC) and Newcrest Mining (ASX: NCM) to name a few. These businesses also have a profound impact on the make-up of the ASX, so the trajectory of the Australian dollar could play a critical role this year in determining the year-end finish for the market.

Investors should pay close attention to the impact of forex movements when 1H21 results begin to emerge over the coming weeks.

How will mega-tech fare in the year ahead?

Tech stocks have proven to be one of the biggest winners' across the market throughout the pandemic, with the theme of digitisation swelling amid lockdown restrictions that saw individuals stay at home and work remotely.

As we've touched on, with vaccines being deployed this year, many countries are hoping to reopen their economies. But will the tech trends that emerged last year remain prevalent throughout 2021? Have we seen a permanent shift in behaviour favouring things like cloud-computing, online collaboration, ecommerce and digital payment technologies like PayPal (NASDAQ: PYPL)?

Given the weight that US tech stocks lent whilst helping the market recover from its March crash, it is likely that these trends will need to continue showing signs of ongoing growth to support the market prospects of ending the year with a strong positive result.

We also have emerging issues in the US surrounding the regulation and taxation of mega-tech companies like Facebook (NASDAQ: FB) and Alphabet (NASDAQ: GOOGL), which will not only sway the US market, but have a flow-on effect for the ASX tech sector as well.

Which companies will be the stories of 2021?

Following last year unpredictable turn of events, the lesson at hand for investors is that one should never write off the impossible.

Many investors dismissed the notion and severity of the pandemic when it was first emerging a year ago, and subsequently, many investors looked past the prospect of the market being able to rebound strongly, even if spurred on by government support.

Regardless of the market course throughout 2021, it worth keeping an eye out for those companies facing headwinds, and those with the potential to bolt from the blue. Last year, it was Afterpay (ASX: APT), which soared nearly 300%. In contrast, stocks like AGL (ASX: AGL) finished the year circling all-time lows.

If last year is anything to go by, the lesson at hand is that spotting the next big trend early enough can pay off handsomely. What will be the next big trends of 2021?