Goal: to slowly wipe out ALL human sims in the town and replace them with an alien society.
(for harder difficulty: reach the top of EVERY career with a different alien)

Before You Start: You WILL need the Seasons expansion pack (duh) for the aliens. I also highly recommend to play with Into The Future because this will unlock a lot more game-play options and make the challenge a lot more fun. Another expansion pack I recommend is University. EP's you do NOT want to play with are Supernatural, World Adventures, and Island Paradise (they screw up the game a lot and will not add anything helpful for the challenge.)

If you are not against using mods, I recommend Story Progression and Tagger by NRAAS.

Rules:
1.) You must start with ONE human sim (preferably a male as they can be impregnated by aliens) as your founder. Any other sims must be moved in.

2.) Aging and free-will must be turned ON! You can choose however long the life-cycles will be, to make it easier/harder/realistic.

3.) You can use ANY method you like to go about reaching the goal, you have the freedom to choose the most creative or outlandish methods you can think of. (These are limited to what you can do in-game. Meaning you CANNOT just bulldoze all the houses and evict all the human families, or move in aliens saved in the bin.)
-You can change the household you play as much as you please (in fact you'll probably need to for breeding purposes) but ONLY alien families.

4.) Aliens CAN reproduce with human sims, but they are not like other occult lifestates where the baby will be born as one or the other. Instead, they are hybrids and their DNA will deteriorate the more a lineage mixes with humans. A hybrid counts as an alien if when they are teen or older they can do the alien exclusive abilities.

5.) Have a Headquarters household. You will find that there will be a lot of moving sims in and out, reproduction, and money to buy or build houses for habitation. Having one household that serves to do all of these things will make it a lot easier.

6.) You can move aliens into households with humans, and play in them too. However, you are not allowed to play any humans or make decisions for them unless they are a spouse (it would be too easy to just move an alien into a human household and move all the humans out – no fun!) You CAN however, move humans in by friendship or marriage (as it is assumed that is their choice) and do whatever.

7.) Please keep a spreadsheet or some kind of notes, you will need it to keep track of all the aliens for pairing and breeding. Keep track of their ages, and separate them by gender.

8.) Human slaves (humans you move in by asking, marrying, or raising from babies) do not count towards the human population and therefore you may finish the challenge with them around. You cannot have more than 2 human slaves per family, this is to discourage cheating.

9.) And on that note, no cheating! The only cheats allowed are testingcheatsenabled, resetsim, and familyfunds (ONLY TO TRANSFER FUNDS between alien families). They are only allowed to fix broken sims, objects, or moodlets. No money cheats, and no Inheritance or Moodlet Manager LTR.

If you have Into The Future:
Once you have at least 6 fully alien households, you may:
Visit the Future
Use Future items and appliances in the main world (not before)
Place Future lots and venues in the main world (MUST be owned and bought/built by the headquarter household.)
Make/buy Plumbots and use them as you wish

Tips!
You can increase the chance of an alien visit by collecting space rocks and using the telescope everyday, as well as having space rocks in your home lot. Collection Helper will be useful!
I found that a lot of the times visiting aliens are in the elder stage. Elder females can't breed but alien visits are rare and crucial for genetic diversity in the early stages of the challenge. If you have generations or choose to play with supernatural, utilize the chemistry and alchemy skills for potions to turn them back into young adults.
Aliens can get extremely boring because they all have the same look. If you're like me and don't like to give your aliens human hair or clothes, use testingcheatsenabled to edit them in CAS to change their skin color and clothes to something cool (I like purples and blues). However, ONLY do this to aliens that are in day 1 of their current life stage, as editing in CAS can reset them to day 1 and would be considered cheating.
I like to make my challenge more interesting by naming all my aliens funky names and buying property to make them more alien/futuristic looking, as well as renaming the lot. I feel it does a lot to contribute to that feeling that the town truly is being invaded, and it's cooler to gauge your progress by how much the town changes.
With the Tagger mod, you can see the location of ALL the sims in town. This makes it easy to gauge exactly how much progress you've made, as well as sniffing out any specific aliens for breeding, or humans to kill. Another use is to see exactly where a visiting alien is at night, so you can quickly show up and befriend them.
If you are using the Tagger mod, make sure to have someone from the Headquarter be acquainted to EVERY alien sim regularly, because the tag color will change. This makes it much easier to find new alien births or visitors you might have missed.
You can make the challenge harder with the Story Progression mod, which will allow humans to continuously reproduce, which makes it difficult to kill all of them and finding empty houses to move in your aliens. (without it, you can easily outlive the humans since they'll all die out naturally)
With Story Progression you can also turn on stories. While it is annoying, it let's you track exactly what is going on with the population. It lets you know every birthday so you can edit them in CAS or use for breeding, as well as deaths, romances, house changes, new immigrants/emigrants, which are all incredibly useful for planning strategy.
Do NOT make aliens breed like crazy early on in the challenge. I know it's probably the first thing you would want to do to rack up the population, but you will find that if you breed every single eligible alien to eachother all the time, eventually they'll all be related and you'll be stuck trying to increase genetic diversity. (this goes with cloning as well, clones will show up as children to the scientist, and if you do too much with one sim then there will also be no genetic diversity, however you can still win with this method.)

NOTE this is a long challenge with a lot of manipulation going on. This means there is a chance of your game being corrupted. Remember to take precautions to preserve it, do not play with EP's you won't use, Save As in a different save file as much as you can, if you are planning to do a lot of building or lot make-overs, do it in another save file instead and just place it from the bin to your game. Overwatch+Error Trap, and try to use the game mechanics as much as you can instead of cheats/mods.

When you are done with the challenge, you can choose another town to invade. (This challenge's format would also work with any life-state! Try invading with plantsims and making the town more naturey, or robots, or vampires with a dark, goth-looking town, etc...)

I think this is it , let me know what you think or if there are any questions! I would love to see everyone's experiences

I like this idea! I don't have ITF as I'm usually not into that kind of futuristic gameplay, but actually your challenge makes me wonder if this expansion can still be aquired :-D

I'm currently stuck with a really unstable 100 babies save, but if I've finished that ooooor if I loose my patience, I will go and try this out (and even buy ITF for that :-D )