New Gear and Crafting Recipes
Naturally with the latest expansion there will be tons of new gear and crafting recipes. Previews of Stormblood have shown off new job exclusive gear and things like this in particular will be very popular and in high demand. As with the last expansion Heavensward, all existing latest endgame gear will likely become obsolete as the new gear topples it. Acquiring the materials and being able to craft the new recipes so that you can sell them on to other players will likely be a very profitable route for crafters.
Furniture for Player Housing
Player housing is a huge gil sink in Final Fantasy XIV and with Stormblood adding even more lands for housing, it's a booming market. Whether you can afford to buy property yourself or not, you can cash in on this. The players that do own houses will want to furnish them, which requires crafting them from materials. You can cash in by farming and selling these materials.
There is some finesse to this in order to maximise your material farming efficiency. Take the time to look in the crafting log for common materials used among the most popular recipes. These are the materials that your time is best spent farming, as players will want lots of them to craft all the furniture items to decorate their home.
Gardening for Gil
Similar to player housing, players with property will be eager to buy items like Oriental Grass Plot and other items, such as Sabotender and Chocobo Topiary, to decorate their gardens. Some of these items can only be acquired through airship missions, so there is a high demand for them but restricted supply. It's not easy to farm for these items, but if you can get them then you should be able to sell them on for a tidy profit.
Alchemy for Potions
Players eager to explore the new content on offer in Stormblood will be in great need of potions, ethers, elixirs, stat buffs and virtually everything else an alchemist can make. As is usual with a new expansion, there will be players eager to rush through the content as quickly as possible, and that means they'll be buying potions and the like in droves rather than taking the time to craft the items themselves. Cash in on this by making sure you have a constant supply of the most popular items available!
Playing the Market
Much like traders in real life, you don't necessarily have to craft any of the items yourself to make money off of them. This method requires you to have a couple million gil to your name already, but you can make very good money flipping items just by watching the Market Board. By buying items while low and selling while high, you can make gil just from exploiting the natural price fluctuations in the market as a result of supply and demand. Tracking this properly in order to consistently make gil from it will require some serious work, utilising spreadsheets to track item highs and lows and identifying which items are likely to rise in price. In principle it's like a simplified version of real life market trading, so if you ever wanted to give trading a go without having to commit real money then you could use this strategy for some practice and carry it over to real life if you're good at it!
The main alternate to crafting that will also be very popular in Stormblood is selling runs. If you have a character sufficiently geared and levelled up, you can sell runs to other players who want to experience the content but can't get through it all themselves. If you can gather together a few skilled individuals and band together to sell your services, you can easily make a killing off of this method.
With the wealth of new content coming in Stormblood, there will no doubt be an influx of players both new and old eager to explore all Stormblood has to offer. There will be players who want to rush all the new content to get there first and there will be plenty of rich players looking to build property in the new plots and furnish them. By anticipating and preparing for what players want, whether these are materials, crafted recipes or runs, you can plan now and be ready when Stormblood launches and demand is at its peak.