Well that's a project. I'm not adding all mods at once, so if it crashes I have a few ideas already. I've tried that numerous times (successfully) but I started over and over before it was finished because of mod updates and all.
The last one before now, I only needed configuration, resource pack (adapting Painterly Pack to the hundreds of added blocks for variety, love that thing) and further testing on the server (only testing in solo until it's mostly finished).
I'm not into magical stuff, so this modpack mostly focuses on expanding the universe (Mo's Creatures, AppleMilkTea, CustomStuff 3...), enabling more contraptions (MineFactory Reloaded, qCraft, ProjectRed...) and sculpting complex buildings (BiblioCraft, Carpenter's Blocks, Armourer's Workshop...).
Also a few utility mods (MagicYarn, CustomNPCs, MultiMine...) to wrap that up in a private (at least for now) multiplayer server, which will consist in creative building cities within a coherent and realistic (for Minecraft) multiverse, then maybe a little roleplay.
I'm thinking of stuff like army bases (Flan's Mod), space stations (Galacticraft), railroad stations (TrainCraft)...
Of course it comes at a price : I'm trying to keep it working within 8 gigs of RAM (Windows OS included), so about 4 to 6 dedicated to Java. It's not really optimized.
I still have loads o'mods to recommend, that's just a preview. If you have interest in a particular gameplay, I might be able to suggest some tailored to your needs. If you're into magic for example, Thaumcraft is a nice start. Never played it though, so don't ask for help, heh.
You'll also probably like premade modpacks like those found on FeedTheBeast. If you need any link you can't find on Google, don't be afraid to ask. Some I got from japanese forums (I don't read japanese, just found through other sites), some from GitHub, but mostly CurseForge and the Minecraft official forum.
That's it for now, if you need anything, you know where to find me