Human Invasion

Funny, that is exactly why I find it interesting. I mean, in most forms of fantasy gaming, those downtrodden and "evil" races that are goblins and kobalds usually have some pretty strong parallels to the issue already. Of course not all settings are like this. But many do feel very classic cowboy vs Indians in that respect.
I feel like flipping it around could be fun since we don't do it nearly as often. I mean, we play AS evil Nazi-esque characters then we do as a Indian parallels.

I don't see how the parallels to the colonial times is a reason not to play this kind of campaign. Roleplaying is here to explore different worlds, places and positions. If the GM is aware of what he's doing (which he definitely seems to be), this kind of campaign can give people a secure platform to expand their horizons regarding those issues.

The idea of humans as parasites expanding and "civilizing" everything in sight is a theme that's explored plenty in Sci-Fi (I mean look at WH40k). Using this template on a DnD-Setting only seems logical and has the potential for not only some meaningful discussion, but also tons of fun and worldbuilding. What does a culture of Ghouls look like? How would a band of Goblins attempt to start a settelment?

I really like this, and I think the real life connections only make this more worthy to pursue

To create a degree of separation between colonialism, a DM can always make the campaign continental rather than intercontinental, with the humans as an up and rising species/kingdom. All it takes is a driven population standing behind a charismatic conqueror and a well functioning government. If colonial powers vs natives strikes too close to home, then look to the expansion of the Roman Empire, or the conquest of Ghenhis Khan or Alexander the Great may be a more palatable source of inspiration.

Many of the greatest stories are told from the perspective of the conquered. What better way to do this then as a party of lizardmen, orcs, minotaurs, kobolds, goblins and other "monsterous" races?

Have to advise caution in any instance where a fantasy culture has a real life analog. It's a common pitfall in fantasy settings where a real world culture is distilled to simple flavor elements. Consider a halfling "Romany" caravan, the entire Druid class, nearly every single instance of anything remotely east Asian*... In this situation you've cast the Pre-Columbian cultures as low-tech, low magic. Which is... fine. But everything I've read about treading this line is do it purposefully and for a reason.

Part of the peril is unavoidable. D&D and fantasy generally draw on folkloric traditions from the world over-- partly because it's easy to sample from true life, partly because the world is so diverse and has generated so much content (history), partly because folkloric tradition is in the (free to use) public domain. It oversimplifies the real cultures it uses much like it simplifies the fantasy creatures into monoliths so uniform all lizardmen/orcs/centaurs share the common alignment. It's simplest. But it can go really very wrong when published.

It might be worth noting that this campaign exists. It's from 2e, Forgotten Realms. It was...

...Problematic. Maztica was a fan-fic trilogy of colonial era central America. More recently, they literally wrote it out of existence.
en.wikipedia.org/wiki/Abeir-Toril#Maztica

I have to ask if the tabaxi, lizardmen, orcs, minotaurs, are going to be their respective cat-like, territorial, often CE, usually CE archetypes? I think one interesting way to approach this campaign, using what already exists in D&D--
I imagine the intelligent races battled to a sort of equilibrium state. Something would be needed to establish the wider world and then the characters within the monstrous society. We don't have many good references for how evil races interact in the wild, but there is ample precedent that they can coexist when kept separate, even ally to form monstrous militia movements under a banner (or antagonist). One reason I always imagined for monstrous races to advance levels in character classes was for 'the game'. To maintain equilibrium the races couldn't go to regular all out war-- weaker numbers would weaken both, it's mutually un-beneficial to war. So each race would train up elite squads to contest claims, disputes, disagreements in direct small-squad opposition or trials. That would give plenty of opportunities for adventures as the PCs train up and establish the world, and explore the power dynamics on the continent before the humans arrive.
The arrivals wouldn't really present a challenge noticeable until they settled, gained a generation, and expanded outward. It would have to take longer, if the human settlers had to expand farmland into grazing territory of a 6HD minotaur culture, for example. The bleeding edge of that expansion would be the adventuring humans. From the monstrous PC perspective, the humans with character levels got there mostly by battling the PCs own people or their neighbors. The PCs would end up at the vanguard of efforts to stop the Human incursions, so by that point of the campaign the PCs would be at the center of action for the whole New World.
I think ultimately it makes an interesting prequel campaign for later stories in the more developed world. It would be easy to have call backs to the earlier campaign in those later adventures and build a persistent world. By that later point, the monstrous races would be pushed back to remote pockets or disappeared completely. Only certain secret locations or ruins would still be undiscovered so you could re-use any of the settings or scenes you used; I'm always a fan of narrative ways to re-use the same terrain over a number of encounters. Perhaps the monstrous races are now plotting their return. Plenty to work with. Maybe that only happens if the PCs fail.

Ultimately I think you should tell the story you want. There's nothing wrong with using the prism of the story to examine real world concepts, philosophy. As was said above, you just need to know who is sitting at your table. There's already space for examining the truly monstrous in a world with torture demons, brain-eating tentacle aliens and monsters that can drain away souls. I don't think anyone will show up and smack the dice out of your hand** if you took a look at history through the fantastic.

* Seriously, this is a thing. en.wikipedia.org/wiki/Oriental_Adventures
** Do you get to keep the outcome of the roll if they do?

In my vision of this campaign, there is a significant difference in the cultures of the people. In terms of technology and military prowess, the two "sides" would be roughly equal. In order to avoid degrading certain cultures, the language used to describe these different people must be carefully crafted.

Very rarely does a culture of an entire race portray itself in a negative light, since many of them are ethnocentric. This holds especially true in times of conflict. The "monstrous" races will only be monstrous to humans, but certainly not to themselves. In fact, they would likely hold each other to high esteem.

It may be tricky for a DM to describe other cultures appropriately. However, by using neutral language or even a positive conversational tone, you can drive your players to accept many things as "normal", no matter how alien something is.

For example, if you are trying to portray lizardfolk's cannibalism in a different light, try having the PC's grandmother cordially invite them over for delicious medium-rare steak. That is going to sound a lot nicer than a horrific scene of butchery of some poor sod in the battlefield.

Likewise, don't describe the orc's tent as bare and shambling. Call it practical and rugged, capable to be dismantled at a moments notice should the chief order a migration. To orcs, human or elven structures must seem unbearably gaudy and unnecessarily permanent.

So I started running this campaign, and had four sessions so far. It is quite fun, but the atrocities of war show through quite a bit. Here are a few things that I am surprised about:

1. Desperate people are willing to do desperate/terrible things. Given the setting, the PCs had no issues committing acts of terrorism, just because a commanding officer told them to. In fact, they didn't even question it when they were told to destroy a lighthouse so that enemy trading and supply vessels would crash into a nearby reef.

2. Following orders is easy. Leading is hard. I gave my players an option to be the captain of a privateer vessel, but none of them wanted to be the leader, and deferred that responsibility to me without a second thought.