Others may chime in of course, but I think Shammy/SK is a brilliant box setup for Agnarr. Slows are meaningful in this content, they have moved the healing spells down a bit (helps druid, shaman and paladin), buffs are powerful in this content. SK are good tanks whereas some of the other tanks need a few expacs to get abilities people otherwise assume they have all along and FD is useful. I won't play either, but I have yet to enjoy being a tank and shaman wasn't fun for me on Quarm.
Enchanter also gets to relive the golden age of charm being super-powered. GoD/OoW is where content was less chanter friendly, and charm got nerfed and so on (much of this was undone post level 75 and enchanter is great again on live). So chanters that get played to 65 on Agnarr will always be good. Chanter is a highly abandoned class though, as there power comes from not being frequently afk or inattentive and I feel a lot of people are drawn by the power but burn out on the responsibility. I've played chanter several times in the past, I will probably have an alt one later on for Agnarr.
Rogue is badly underpowered for much of Agnarr's progression. I found the Phinny playerbase for PuGs pretty much took rogues last. AA start to help, and gear into PoP is much better. I won't play a rogue again on progression, but if a person loves being a rogue I think it is a viable choice.
Wizard is a great debate at the moment on the official boards, with more people seeming to feel the class is under-developed at the start and needs PoP to be competitive. There is a strong "anti afk wizard" voice so take it all with a grain of salt. I'm debating a wizard main, because I do logic like that sometimes.
Magicians are strong in group content, with pets heavily nerfed in raid content for several expacs. I may play a magician past level 14 for the first time...
Paladins were pretty rare on Phinny, warriors always in demand for PuGs. I won't play a tank.
Rangers become insanely powerful by the PoP lock of Agnarr, and are a viable tank in the early expacs (though many won't believe it, but if one is just looking at the math and abilities given to tank classes original-Kunark and doing average group content...). I always have ranger alts.
Monk gets overplayed early on progression servers because people think the fists are massively overpowered (they once were on progression), they are still stronger than original of course. Monks are a good if not great DPS class for Agnarr, but you will be expected to pull by most groups unless a bard main that likes to pull is in the mix. I may main a monk for the first time.
Cleric --everybody wants other people to main a cleric. Lots of "true boxes" will be clerics. PuG groups on Phinny saw some people use their cleric status as an opportunity to be a brat, but most were good people. As Luclin lingered on it was interesting to see some clerics LFG in longer stretches vs. logging in and insta getting a group --but this may well have been server reputation beginning to pay back karma. I haven't played a cleric since LDoN was first launched... I may main one to start Agnarr.
Necromancer will be a solo king on Agnarr because the XP isn't crazy nerfed like it was on Phinny. They can be good in groups too, but many don't see it (including a lot of the necro player base). Into Kunark necros can mez most group content, have healing spells, debuffs, slow undead, charm undead, nukes, a pet, and in a skilled players hands are one of the best pullers in the game. They are also not gear dependent to get rolling and can solo anytime, including while the rest of the group is medding. Some guilds think necros are twitch bots, which just means they aren't using their necros well. I've played necro too much on Live and past progression servers, so I won't be rolling one on Agnarr to start if at all.
Druids will be common for the no pok travel era of Agnarr. Some of these druids will be people selling ports to buy Krono to take back to their home server. Since they fixed the dot-changes fix in time for Agnarr, they should be jack-of-all-trades and solo viable again. In pug groups expected to be recruited as a healer. I like having a druid to buff newbies for fun, but I doubt I bother playing one on Agnarr (my older druid got boring for me in the 50s).
Beastlord is so strong for their leveling in Luclin, and fun to play 55+ with lots of AA. They are more of a group (or even solo) class in most people's mind for the Agnarr lock with a limited raid role and therefore limited raid slots set aside for them (in guilds that do that). I'll level a bst alt, with an outside chance of it becoming a main. They are fun.
Bard will be a common box on autofollow singing /melody. They are good in groups if not excellent. They will be wanted for pulling but are also good at CC (with some hiccups with level cap stuff at some points). A truly-played bard main will be a thing of beauty on this server. Many old bards haven't forgiven the devs for nerfing so much of the solo tricks though, so hard to say. I've never played one, maybe an alt on Agnarr for me.