1. Language. Language should definitely be more mixed around.
First hand, grammar and vocabulary. Brains do that for expediency, at the cost of making educators and bureaucrats wanting to kill themselves, if the point gets across, then what's the problem?
Especially to communicate hard expressions when english lacks the word. Like for example, 乖 . Well, I could explain to you what it means - a form of praise for obedience and being filial.
you can use it in terms of an adjective > he's a very 乖 child, or a verb by itself, 乖 to praise someone
or you can just use the word by itself instead of going through translation spaghetti.
Similarly, linguistic habits. These are more from enviroment - like how in people say "ouch" differently - personally, I say"AIYAH" Kinda weird, but that's how I was raised.
It can also be as simple as the slow shift from people saying "Pardon me" to "Excuse me" and "You're welcome" to "No problem". These things you don't notice until you've realized they've already happened.
Easily explained is how American English has drifted from British English - z's and s's getting swapped, u's being removed, etc.
In all honesty, it comes from mistakes your brain makes in the neverending quest to explain things to people.
2. Food
To be honest, Fusion food doesn't have to be like, whoa slam two foods together - It's more like adapting to a culture. Japanese burgers and pizzas are prime examples. They love their bellpeppers and barbecue sauce, and their beef burgers aren't 100% beef. they knead in some vegetables and stuff in there too ( literally the only reason i know this is because i bit into a mos burger and got a bad surprise ) They don't fundementally change the thing - like changing the buns to some weird wakame blobby mess or a pizza made with rice ( please don't try to give me IRL examples to prove me wrong i'm just trying to make a point here ) but just the topping - little, tiny changes that give it a little touch of culture.
Just to complement their diet. I love unathi food. They've basically taken human food and said "needs more meat". Which 100% makes sense.
3. The Homefront
I feel like the loredevs have been focusing too much on the politics of some far off planet I don't really care about - I'd much hear about stories of people's daily lives. The working class, their families - you know, people that we're going to interact with the most. Like a Tajaran mother having dinner - how is she serving stew to all her scamps? A quiet dinner meal at an Unathi household - tense, where the son doesn't want to take over the father's family business, or how some Skrell Drifter eats his Microwave Dinner Tenta-crisps watching corny Skrellian romcoms. I feel like this'll help roleplay more - In real life, we don't really talk about politics, do we? We usually talk about our families, our lives and our dreams.
Why not in our lore too? What's a usual day in these planets? Their cities? How's it like to live there - Be born and die there?