Nothing of consequence gets skipped by starting in kunark. Oot goblins and maybe 3 total quests like the aegis if life, only horrific stuff. Kunark zones are also wildly nerfed xp from 30+ and hard to reach before that so the classic zones people DO like should still be quite viable (unrest guk etc). In fact, I think a traditional "fast" group with mage cleric enchanter is likely not even going to kunark until the 40s!
What DOES change a lot is the optimal class situation. The large zone access makes "ahead of the curve" a lot less meaningful, and instead, that 50-60 stretch is where the real slog is. A shaman is arguably better than a mage for your malo caster. A necromancer or even druid is far better than a 2nd enchanter once you already have one. In fact, the "fastest" groups will probably want to go really hard on either multiple necros or multiple druids, since you can now level in 100% undead areas that are decent, or 100% animal (often insect) areas that are usually decent. In a classic only start, a druid can do alright until goblins but sucks in sola, so you're stuck in insect spots and NEED efreeti camp while being outpaced by groups in better zones, whereas here they can charm in places like kaesora and droga and seb and then are stronger than enchanter in the chardok home stretch. Necros are even better here.