Firstly, dungeons are not really focus of endgame anymore; instead people focus on Fractals as 5-player endgame content. Raids require expansions anyway, so no need to focus on those here. Secondly, frankly there are no core builds wanted in endgame of instanced PvE content.
The design idea of elite specs is to improve/add to one core aspect of the profession (aka class). Therefore, whatever role your profession could fill, an elite spec can do it even better. Frankly, nobody sane would explicitely want a core profession as a party member in Fractals. As such, there is no really good/viable core build for Fractals.
In case of GW2 you should consider expansions more of buying the game overall. PoF even includes HoT nowadays. If you want to experience the game in any proper way, you would want at least PoF; nowdays properly even EoD as well.
Crafting professions are required for crafted endgame (ascended/legendary) gear. Therefore, they are overall important, yes. They also allow generating money via e.g. crafting time gated materials and selling those. You can also get ascended gear via almost all kinds of endgame content nowadays, but that requires quite some (time) investment, which is why crafting still matters for gearing quite alot.