There is no way for bitcoin not to be deflationary in the long run.
First, consider that inflation and deflation of currency is the opposite what most people want to happen to bitcoin right now - which is still counterintuitive, because its adoption as a money is counter to its value as an investment. If the cost of a bitcoin rises against fiat, IE, the price goes up, then the currency is monetarily deflating - it is getting more valuable over time. Likewise, when the price drops - ie, after the 2013 bubble until the start of 2015 - thats inflating, because your money is worth less over time.
For pretty much every fiat currency there is, your money is always worth less over time. It means you don't actually want to have physical money, because that money is all kinds of things you could have instead that either appreciate in value or give you a value add versus the money just losing its purchasing power.
In almost all cases that inflation is due to a growing monetary base. States are printing more fiat money all the time, and sometimes they do major injections of money (quantitative easing, for example) to stimulate economic growth. That devalues the money, and it means you don't want to have any money.
That mechanism is good for society, because it means that nobody is just hoarding a gold pile somewhere. Every billionaire is still spending almost every cent they have because to not spend it is to lose value. The last thirty years since the removal of the gold standard, the USD has seen rise to an insane turnover rate, where monetary velocity is incredibly high because nobody wants to have USD for long. You turn it into shares, you turn it into physical assets, you turn it into salaries, you do something with it that is not letting its value rot in a vault.
With bitcoin, varying design constraints compound the issue that unlike with fiat, and like with a scarce resource like gold, hoarding it makes perfectly good sense. Bitcoins monetary base has constant velocity for years, and its only decelerating. Every four years the payout rate of mining a block is halved. Its already been halved once, and will be halved again next year. It keeps getting halved until mining stops paying out at all in about a hundred years. But because you are halving the payout rates, there are already over 14 million btc out a limit of 21 million, so the monetary base is already over 2/3's as large as it is ever going to get.
That kind of base cannot handle anything catastrophic to its monetary system. A sudden influx of investment into bitcoin will always skyrocket the price. Whenever it has a breakout moment the price goes insane, and there is nothing to price control it because there is no way to expand or contract the monetary base to account. As a result, I and many other people are just sitting on bitcoin hoards waiting for Amazon to start accepting them so that the price goes to Mars again.
That kind of thinking, and that kind of economic interaction, is a huge backslide of a common currency from what we have today. The best money is one you don't hold onto for long, that you want to get rid of as quickly as possible, because it maximizes monetary velocity which means your economy is running at max speed. If you slow down velocity - if people stop spending money, and it stops changing hands, as is often the case during uncertain political environments - your economy slows down. If you put bitcoin in that position, there is a huge incentive to not spend bitcoin because bitcoin is a mathematically limited resource. The more its used the more valuable it gets and the more pervasive it is the more reason there is to never use it for its "intended" function.
Its the perfect gold replacement. You could strike a gold vein today and be a millionaire overnight. Unless you can impersonate a wallet (basically a sha1 hash, good luck brute forcing that) you aren't going to find random gold. And that isn't even digging up new gold so much as its finding gold in someones basement, maybe in a vault you broke into or maybe buried behind a wall long forgotten because the owner lost the wallet or password. Its even better because bitcoin is literally only the value of electricity and computer hardware that make up its network. Its a lot less distortionary to goods markets than treating gold, silver, platinum, etc as scarce commodities is, because it means people buy and hoard gold and other precious metals just for the sake of hoarding them, which drives the price up for anyone who has a practical use for that metal (gold is a great conductor, and gold capacitors are super efficient) making it prohibitive to use because everyone is valuing the metal not for its practical utility but for its rarity. The commodity market for palladium is no where near as ridiculous as golds, even though they are both rare earth metals, because gold has a history that makes people buy and own it for literally no good reason besides other people do. And you could do that exact same mutually respected scarcity == rarity == value proposition with bitcoin, without any ugly side effects like making my computer slower because putting gold in half my circuits would double or triple the cost of the product at least.
Of note most altcoins have no monetary base caps. Litecoin is basically a slightly improved forked bitcoin, so it still does. But currencies like peercoin and dogecoin have no caps, but do it in different ways - peercoin has a static 1% inflation rate, so its constantly minting more peercoin over time, but the same amount of the base is minted annually - which means if you are losing more than 1% of the monetary base each year due to lost wallets, your currency is still deflating. Dogecoin statically mints 5.25 billion coins a year, so eventually your monetary base becomes large enough that you are losing as many coins as you are minting.
What I'm getting at is that no cryptocurrency yet has actually solved this problem, and I've already posted my own ideas on it all over the place, but its really the breakout problem domain to getting a viable money replacement for average joes rather than a gold replacement.