@Ganymede It depends on the mods. Some can be added to existing games, some can't. My advice? Play it out a while until you feel like starting over/you win/you die. And for your -next- colony, you can add mods. I'll list some mods here that I like, so everyone can poke at them.
(Stars * indicate complexity. One being the least)
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Facial Stuff - Face/Beard/Skintone options. You can even set how balding you want your pawns to be!
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Chickens Multiproducer - You can set your colonists to make x5 or x10 of things, like meals. Super handy. Makes the game a bit easier, though.
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MinifyEverything - Lets you minify/uninstall -everything-. Makes the game much, much easier but I love it because it lets me re-arrange my base.
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Increased Stack - Lets you stack things up to 10x larger. Great for saving space. Makes the game much easier.
** Dubs Paint Shop - Lets you repaint/recolor anything. Purely cosmetic. But cool.
- Pick Up And Haul - Pawns will gather things into their inventory to haul. It lets them carry various/more things per trip, as long as they don't reach their weight limit.
*** RealRuins - Lets you discover ruins of other people's previous colonies. I -highly suggest- disabling ruins at start locations, and just visiting ruins that pop up on your map. Super awesome. I love seeing what other people have built.
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Share The Load - Lets multiple people bring materials to blueprints. Why can't Bob help Bill out by carrying some wood for him?!
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Research Pal - This one is a HUGE HELP. It is purely cosmetic, but makes the research table so much clearer/easier to read. -A must have- if you use any mods to expand the research tree as things get so jumbled otherwise.
** Hospitality - Lets you host guests. They can stay in guest quarters and you can designate shopping/recreation areas for them. You can even recruit them, potentially.
** GlitterTech - Adds a lot of late game buildables.
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InventoryTab - Adds an Inventory tab that lets you locate items easier.
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GloomyFurniture - Okay, I'm a nerd, but I love this one because it lets me build small, cozy cottage type furniture.
** Awesome Inventory - Lets you have an RPG style inventory for your pawns. You can see where they are wearing what. As opposed to a list.
- Smart Speed - Adds another speed setting, even faster than the fastest. Good if you want to just push through the dead times.
** Replace Stuff - Lets you just build over existing buildings, rather than needing to deconstruct them first. (CAUSES MAJOR ISSUES WITH REALRUINS IF YOU HAVE RUINS ON YOUR STARTING MAP)
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CM Color Coded Mood Bar - Makes the mood bar color coded, so you can see who is in a good/bad mood easier.
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Power Alerts - Lets you know if your power grid needs attention. Disconnected generators, etcetera.
**** Dubs Bad Hygiene - Makes your pawns need plumbing/drinking water/toilets/showers. Very advanced wizbiz.
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My Little Planet - Lets you generate much smaller planets. I always use this now.
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[O21] Kakamora - Adds the Kakamora from Moana to the game. Essential. Much immersion. Very wow.
This isn't my whole mod list. Just some fun stuff I think really enhances the game. Honorable mentions include A Rimworld of Magic (adds D&D style magic), Call of Cthulu - Cults (Lets you form cults and worship Elder Gods), Rim of Madness - Vampires (Adds WoD vampire clans), and Brotherhood Compendium (Adds the Brotherhood of Steel).
Those really, really change the game a lot. And unbalance the crap out of it. But having a lich or a Malkavian vampire or calling on Nyarlathotep for aid really is fun.