What will Christianity evolve into over the next few decades?

An increasingly small cult that will be viewed by normal people as a whacked-out fringe group. 

The original and true Christian Church will continue to teach the fullness of God's truth, just as its founder, Jesus Christ, said it would, just as it has been doing for the past 2,000 years, while Protestantism will continue to fragment into more and more conflicting denominations, as it has been doing since it came into existence a few hundred years ago.

Christianity has been the same for over 2000+.  

Christianity will decrease more and more, while Atheism will Increase more and more.

The future belongs to us. Jesus said so. So if you are anti-Christian and looking for a better world where you can burn and pillage with abandon, you might get it, but it won't last long. Christ is returning, and there is nothing that will stop him. 

The church of Jesus Christ will not change in any material way.  Its doctrines are pretty well set, barring new revelation, and the standards that Jesus Christ teaches are still relevant.  Human nature has not changed in the last 2,000 years.

Mainstream Christianity?  There's no telling what will happen to it.  I think the Beast and the false prophet will be amazingly successful at drawing Christians to them.

I hope if it’s God’s plan for us that we return to the way of the early church - having house churches (with visits from apostles who are filled with the Holy Spirit, so there’s accountability) - smaller, closer relationship and community care for each other. No more expensive buildings, but maybe hiring places every now and then for house churches to get together with the other house churches in the area to worship the Lord.

There has been a 'world-altering' Christian revival in each century that has reshaped the secular world each time it occurs, suddenly changing everything in unimaginable ways before eventually 'normalizing' and being taken for granted.

Each of the major Christian denominations became recognized by a similar revival, which is how they each went from being upstart movements to fully recognized denominations. The last one was in 1906, ('The Pentecostal' movement), and can even be traced to a town in Missouri and particular street called 'Azusa street' where the phenomena first began.

The usual pattern is for the effects and influence of the revival to begin to wane in the country of origin while continuing strong in the developing world, just in time for the next one to begin. Each time questions like yours get asked because there is always something major happening in the world to challenge Christianity that seems invincible in some way. That challenge is typically the first thing to fall away and give way to the new revival and inspiration.

I am already seeing signs that the next great Christian revival will be among the Messianic Jewish Christian Church, which was actually the original Christian Church formed on the day of Pentecost as recorded in Acts 2. 

Large gatherings will be back.