(I like the idea of Monster Hunters with mechs. Makes me think of fantasy Lost Planet. I'd play that)
I'm having a hard time to justify it solely as a mech monster hunter rtd, since there would be little progression in regards to world development because monsters don't have hidden agenda's and the futuristic elements nullify the need. They just do what their instincts tell them to. Basically, you'd get a contract from someone, gather some information and prepare gear, find the beast, slay it in varying methods, bring back proof of succession, get mad scrilla, repeat. The monsters would get increasingly harder, but you'd get increasingly stronger in return where eventually you'd be like high end wow characters, coated in colorful armor with blade of mass doom +7 and thermite cannon +3.
There can be subplots where someone is manipulating monsters, or natural events but you can use that only so many times before it gets stale. While human to human or alien race to human conflict is much more broad and interesting to describe as the reasons can vary from simple war for turf to political struggle to assassination attempts and the prevention of that.
Anyhow, I'm combining some elements and there should be a thread up soon.
So at best, it would be a mech rtd. Where you can hunt monsters for bounties.
I noticed you spoke of aliens. I think plot problems depend of what will be the definition of Monsters in your RTD.
Will they be natural creatures only following their instincts as you said ?
Or will they be natural sentient creatures with a hidden agenda ?
Or you could even make those monsters "mechs" for the alien faction. They would be bio-engineered war machines controlled by aliens for a rough example. Just with that, you could do loooot of things.
I feel like I'm throwing ideas without thinking.
When I think of a monster hunter, I think of someone hunting natural beasts. Like hunting lions, elephants,etc. Just local Fauna on some random planet and as it can happen, people, or someone else can work on a planet and suddenly some monsters come pestering them. Though instead of lions, you get massive 16m tall spined worms which spit corrosive acid and have 22 eyes.
If you were to fight vat grown monsters, then this would strictly mean that you are fighting against an other race. Whilst you may be indeed be killing monsters, you are doing it in purpose to stop the others from making any more. So you are at war, not hunting.
Hunting is just war against nature.
Let's say that the race that made these monsters (be they vat-grown abominations of science, rogue robots, natural fauna infected with grey goo nanites, or even just Ridiculously Hostile Wildlife) has since died out, possibly at the hand of their creations, and now humans have found this natural world rich with natural resources, alien technology, and habitable biosphere that has a teeny tiny rampaging monster problem. So, the government (whether a United Nations of Sol type deal or whatever Megacorp is trying to exploit the planet) decides they want to kill all the monsters, ideally to extinction.
Now, let's also say that something the scale of total planetary genocide is beyond the abilities of this government, either because they lack the funds or knowhow to do it or because the phrase "total planetary genocide" is horrible PR. Instead, they institute a bounty system; the settlers, workers, and freelance mercenaries on the planet can make a lucrative living fighting these horrible monsters for cash, and the government doesn't have to use any resources beyond cash money or be seen murdering alien babies. On top of a standard "per-head" basis, the government also periodically has exclusive contracts which can be bid upon by the more organized or well-equipped bounty hunters; stuff like "we found the lair of the Queen of the Ravenous Fire Spidants, go take her out," or "we learned that the Steelflayer swarms seem to converge on this one plain every few years to breed, escort these scientists so we can study and eradicate them," or "our archaeologists have found what appears to be an ancient alien factory, go look for tech, mind the Rage Mantis hive." And that's just as far as the "Man vs Nature" conflict goes; competition for these contracts (especially the one-offs) could foster hostility and outright bloodshed between rival bounty hunters, monster rights activists could show up to protest and perhaps violently oppose the bounty hunting, the extinct alien creators could be not-so-extinct, yadda yadda yadda.
Teal Deer: hunting monsters is only as boring as you make it