I'll be the first to admit, I'm very sensitive when games that technically have solo play try to force me into group play. No, I don't want multiplayer, and no I'm not going to like it if I'll just try your version of it.
Anyway.
Looking at the website info Horizons and seasons, I see a lot of stuff geared towards multiplayer...wings...multicrew ships...etc.
So what is new for me? What will I be "locked out of" as a soloer?
I've tried reading the info on the official site...but lets be honest, the presentation of that info is terribad.
I'm not a hater or a troll...I want to want it.
As others said, Planetary Landings (incl. Trading or Mission Destinations on Planets) and the sometimes very nice scenery.
Other than that, to be honest your Question will be impossible to answer until end of the year.
If last year was a guide, don't expect anything spectacular for Solo Play (meaning either Solo Mode or simply playing "solo" a.k.a. alone in any other Mode).
We still see NPCs being bugged, getting stuck & blown up in Stations or do weird things. I mean often
absolute basics, sometimes being fundamental issues >1 year old.
Can't quite imagine that we're soon getting a huge surprise-overhaul with fully funcational NPC Wings and/or NPC Multicrew.
I just don't expect that.
Thus, I think waiting it out won't do any harm. Worst-case you'll be picking it up at a discount and far better developed than what we have now (then at V2.4 or V2.5 maybe?).
Apart from the mentioned things - you're
currently not really missing out on anything.
Just keep an eye on the major Updates, watch a few Player Videos and re-assess if you find something interesting for your playstyle.