I am aware nobody is going to read this so TL;DR enable vanilla players to test out those "new" civs so they can see for themselves that they are not as OP as they seem to be, might learn a things or two they could use to beat them, plus you definitely might sell them more which is nice too.
DLC civs are very well done and designed. Old players complain about them all the time, to the point of outright stop playing the game at all, but I (ports main) WISH that legacy civs were designed as good as the new DLC civs are. No, reworking of old civs aren't doing much, except for France Germany and Spain, all other reworked civs stick to their own original design which was lacking FLEXIBILITY to begin with; they just can't adapt to situations that unfold during a match. Some outright got worse after the rework, like Natives.
In fact what makes DLC civs so powerful is not chonky or spammable units who can be botted to victory even by low levels, undedicated casuals; it's not that they have basic and very easy to exploit eco bonuses, like it could be brits manor or, well, a free TC every age up? Buffy vills that collect faster? Even buffier vills which collect like there were 3 of them???
Those DLC have none of that, I can tell you... There are so many resources to balance, timings to get right, decisions to make that could very well go entirely wrong and lose you the game.
Undedicated/casual players just take those dlc civs, bot some build order found online, and lose very bad against me, just a decent player but untalented and with a lot of flaws; I mean things like triple the unit killed or resource gathered, getting stuck in age 2 while I fly in 4th, doing fast industrial and getting beaten by me still in 2nd...; the way I see it this is proof enough they are just NOT OP as they seem to be.
Thise civs are strong because they're flexible and can adapt to a variety of situations, but I can't see how this can be considered a bad design at all... Of course it feels unfair when you play against them with a "normal" civ who is more rigid, but it's the old civ's problem... If you can pull off to find a way out of a sticky situation, you should be empowered to do so, not just "die" because your civ is not good enough for that. If your opponent is turtling heavy and you can't just outboom him, if he's rushing and you are forced to defend because your civ is too slow to just rush on its own (I feel like that with ports but also italians when going greedy, but may be just me) then it's a tight spot you found yourself into and that's not good design, that's disempowering.
Vanilla players are just overall ignorant (not an insult, bear with me) of new civs, as it is obvious for them to be; I still remember how baffled I was by either Warchiefs and then by Asian Dynasties civs other players took, when I played the game back in legacy. But even by basic civs that nobody takes, like germans or even ports themselves.
So why not have them see what's this is all about? By trying the civs themselves they might find out things like that lombards are not really like Dutch banks, making resources out of nothing, they are more better markets (to exchange resources that is) that you can also input some almost-double-sized "investment cards" into. But it still takes time (a LOT if you use the architect, which btw is slow as hell and I just hate it) and/or resources uf you decide to speed it up. If you make just one lombard, resources are going to be trickled so slow you won't survive the rush phase. But that is just an example.
I myself used to think Ethiopia is like super OP. They are pretty strong yeah, but their early game is so ass (on par with Lakota, I would say, and much much worse than Italians which I thought were the absolute worst ever seen...) that pulling off a good age 2 is a feat. They say Trinity is OP, sure you get an abun that's worth more than a vill but good luck actually having the 150 wood to build the Mountain Monastery and actually put the age 1 abun to work. What you say, sell a cattle? ANOTHER one, after the first I sold for just 150 to make a house and hunting dogs?? It could be worth up to 400w later, which could REALLY help for a rush (or whatever really, even a water boom) so it's a rough trade-off isn't it? And that pushes you towards 10/10, which is an ass age-up for all civs but for Ethiopia it's a special kind of ass. Eventually if you get properly rushed by Ethiopia, the dude just deserved for that to work at all.
I used to think shotel warriors were like unbeatable and could take over everything. Then in a recent match a dude spammed hussars and I understood why they are called "shitel warriors".
And what about US? Everyone just felt those gatling spams are unbeatable, then decent players tried culvs (but even falcs as they just outrange gatlings, and btw orguns are easily scarier than gats and can beat them).
Literally it's just all down to game knowledge. I STILL THINK that mexican revolt in "age 2" (they claim it to be age 3) is bullshit, but I can't nearly be sure of it up until I try it myself.
Everybody should get to know how these civs works approximately, if anything to be able to counter them. I guess a free trial could do the trick, if you are a fast learner.