I am building a metropolis too. 8x8 large tiles. Only I am using GIGS of mods and a natural growth historical approach, so mine isnt really a "Fast and furious" way to reach over 1,000,000, but i'm planning on definetely more than that. Also the mods I use are mostly realism oriented, buildings, fixes, NAM, CAM, MMPs...
My advice to Stellaris and everyone if I can contribute is, whatever playing style or goal, to ENJOY THE GAME. If you bought it and joined this forum maybe you like it a little bit, so why rush into building megalopolis with trillion people in it and a vast ocean of skyscrapers when you can enjoy taking your time to shape the scenario?
I see SC4 as a train set model or a diorama or whatever you call them. People spend months working on single trees and grains of sands to make a little hill. It takes patience, yes, and after hours you look at it and it doesnt evoke anything other than frustration. But then you compare pictures a week later and your jaw drops and you feel fuzzy inside.
I checked my own pictures of the first post of my journal and compared them with the ones i took the last few days and... oh my god! These couple of months have really been worth it. I can start to feel the city alive, breathing, talking, asking me. It's not about the numbers. It's about the feel.
Stellaris88 you repeatedly said that SC4 cities look ugly and that you don't want to spoil your city with mods. Ok. To advice you I can say that you might try to avoid the "grid" and I mean trying to avoid to make neighbourhoods that look like a grid, like an infinite picture of suburb LA or Manhattan. On paper they look really boring, but of course in real life they have a variety of skyscrapers and monuments and the roads themselves have several different functions and styles, from alleys to massive avenues to expressways to urban interstates.
Try first to adopt a non-flat map. Terrain is the NUMBER ONE factor in the uglyness department. My first metropolis project was a DIY flatmap and no matter how creative I was with NAM RHW and NWM avenues it just look stale. Browse the internet and find a good map that fits your taste, one with hills and woods, or maybe a bay. A mountain pass. A coastal map. Make your own (although more patience needed). My metropolis named Branny i took the map from the internet, it's the estuary of a big river in a bay. Really sparked my imagination. There are hills although not very pronounced and there is just a hint of a mountain range in the southwest. I adopted a mediterranean climate overhaul and downloaded lots of MMPs and refined my painter skills (ahahah). Even without laying a single road or building the map is already beautiful to look at. You can do this without mods. Just get a good map that doesnt look boring or ugly and spend some time giving it character BEFORE you lay roads.
When you get to start building, dont just lay endless grids or RCI. Use a little bit of common sense and place things where they have a function. Use your imagination. Cities look good if they are realistic. A endless ocean of industry in a grid just looks like a copy and paste effort. Invent. Maybe over there has been opened a coal mine. Put a rail line close to it and connect it to an industry park somewhere else and build a coal power plant. Build also a warehouse and leave space for coal deposits so trains can unload and go back to the mine. You build a port? Then terraform the land around it to ease naval traffic and to build naval docks and shipyards. Connect everything with rail. Make a rail siding with several tracks and warehouses around it. These kind of things give life to the aestathics of the city, makes it look more believable. With or without mods. Obviously there are mods that add buldings, MMPs, puzzle pieces and kits, all sorts of things you can use to embellish or give character or to add functionality. But all of this can be done without mods. I used to. Just remember that the main factor in uglyness is the "grid".
Hope it helps!