- what’s on it now?
- what is wrong with what is on it now?
- what do you do with it?
- what do you want to do different or better with it?
- how much do you want to spend?
Obvious options include wheels (lighter and/or stronger) and fork (lighter and more adjustable). Beyond that things like chain set, brakes etc are usually done for a little bit of weight and for extra bling.
My Cube hardtail is still running all the original components save for the wheels, which I recently replaced to get wider rims. Oh and the saddle is a replacement as the original wore out.
Good point on the tyres and especially tubeless. Best thing I ever did. I used to regularly get punctures on tubes and after going tubeless over 2 years ago not only did I notice the reduced weight but I have never had a tyre-related stop whilst out on the bike. In fact the day I first rode tubeless was the day I got my first and only Strava KOM.... for whatever that’s worth.