I want to go on another solo trip but I'm overwhelmed by options

Hey op if youre a gane of thrones fan theres akways croata

I'd probably opt for somewhere in September (e.g. West Coast NA), not too cold and flights are pretty cheap. You can then enjoy the holidays with friends or family locally, nothing wrong with staying home for the holidays!

if you are going in the conventional winter, then it probably makes sense to select locations that have favorable climates during that time of the year. You could reasonably go to any of these places if you really wanted to, as none of them are made inaccessible during winter, however you ought to rule out london, taiwan, and west coast us due to potential for rain or less favorable conditions. I live in portland and believe me, it is a completely different experience during the summer than winter.

Flights tend to get more expensive during holidays but not necessarily- I have found cheap flights on new years for example, my reasoning being that people were traveling around then but not on the holiday itself. I like the idea of you going to Colombia or Mexico, both will add more variety since you have tasted europe already. Since you are not sure about destinations, know that both countries have plenty to see with a lot of diversity across them, and yet you can book transportation affordably with relatively little time in advance. You wouldnt need to know exactly what you want to see or where you want to go, you can just play it by ear. I am also curious as to why southeast asia isnt on your list for the same reasons. I would use skyscanner to get a broad idea for what prices might be like to these various countries if it helps but I optimistically believe you could get to any of them on a reasonable budget if you can flex your day or departure a little.