This is advice for before leaguestart, rather than after it. It isn’t a build guide, although I intend to produce one for Glacial Cascade Mines and possibly something else. It’s the first step to building your own.
Unless you are Ghudda or Jousis, the first step in designing a build is usually to find a starting point – a previously proven build that has many similarities to the build you want to play, then treat that build as a template to modify.
This can be done unless a new interaction completely transforms the way a skill plays (e.g. when Archmage was added, non-Archmage builds couldn’t transform to Archmages easily)
The single best resource for this is the build indexer at the website poe.ninja
It keeps downloads of the top builds – both by character level achieved, and also by solo delve depth – from the most recent league. It tracks trade league, SSF, HC and HC SSF.
Importantly, it also keeps older versions from early in the league, when items are scarce and character power is weak.
You can get useful information from looking at what worked in both the league ruleset you wish to play, but also from stricter ones (e.g. if you intend to play trade-enabled hardcore, also check out HC SSF)
If you intend to play a brand new skill, you’ll need to look at the most similar skill gems that actually exist now. For example, Crackling Lance isn’t Arc, but it’s closer to Arc than anything else.
This will help you discover the most popular combinations of support gems, Ascendancy choices and overall ‘textures’ of passive trees.
I highly recommend looking at the historical data from days 2-6 of the league. At this point, even top end players won’t have bonkers gear – they are likely running 5 links or bad 6 links.
Builds that reach 500 Delve depth over a league in Hardcore are especially solid choices to investigate, although if you play SC you should definitely adapt them to do more damage. Likewise, HC SSF builds with multiple boss uniques (e.g. Crown of the Inward Eye on the same character as a Watcher’s Eye) indicate a very strong bosskiller.