Oh!!
I always forget to mention the least fun mods of all, "longevity."
IIRC Subaru has not updated its oil pickup from its old, very poor, design. If you dont know how engines work, just think of the oil pickup as the straw that sucks oil for the whole engine. The straw has a habit of breaking at the top, meaning very suddenly the whole enginel isn't getting any oil. What's the actual failure rate of these? I don't think anyone REALLY knows, but there are lots of cases. In reality its probably like .1% but a new pickup is ~$100-200 and for me thats worth not getting stranded somewhere and having to pay for a new engine. Granted, in the scheme of things .1%*$5k for an engine is technically less than the pickup, but whatever.
Also to consider, for long term engine health:
Gauges to, you know, monitor things. Boost is the most "fun" but probably least useful to monitor things (vs AFR, oil temp/press, EGT, etc...)
Air Oil Separator (lots of blowby is common and not good, but not "fatal")
Ummmm, i'm sure there are more.
And then there are performance/looks mods of course. And by this I mean wheels. That's been the first thing i've done to literally every car I've owned. They do technically (usually) offer a performance benefit, but the REAL gains are in looks and personalization. You can get wing stiffies, carbon fiber bits, etc...
And then there's my personal favorite: weight loss! There's usually a few things you can remove without noticing, for free. After that you're generally better off spending $100 on power vs weight reduction in the power-to-weight ratio equation, but weight makes you go around corners faster, etc... Wheels can save weight if you do it right...hence the benefit mentioned above. I've done CF for my trunk, deleted the wing, except for track days, gone to JDM bumper beams, etc...
Anyways, I hope that's food for thought!