From an engineering perspective

[Furan]

First of all, massive congratulation on this fenomenal project!
I work in academia and I see a great potential for Factorio in promoting STEM fields. I am now past the point of a really pathological addiction, I finished a very hard game after 70 hours, extremely limited resources and 800 chopped trees. I haven't really touched the flying robot tier or any of the latest additions.
The thing is I best enjoyed the game from the start (actually repeating great many times) as a survival horror. With almost empty hands (should be an option), without a safe area, lots of enemies, very little resources. Barricading with wooden boxes and tubes. Sneaking deep into enemy territory for resources.
Factorio touches a very important (and actual!) subject of how the intelligence is fragile and needs protection before it can dominate.

Speaking of which, I will share with you one very beautiful gem:
>>>AAAMAAMAAAAEAwYAAAAEAAAAY29hbAICAgoAAABjb3BwZXItb3JlAQICCQAAAGNydWRlLW9pbAICAgoAAABlbmVteS1iYXNlAwICCAAAAGl
yb24tb3JlAQICBQAAAHN0b25lAwEC+LRQ2lGYAABVdwAAAAAAAAAAAAAAABgRwNU=<<<
There's a tiny iron mine for some basic technology and off you go into the wild. It's quite a challenge.
Oh, by the way, run

Now to some random ideas and comments:
1) You can't make an electric engine out of a combustion engine. (As it does not seem to be a generator/motor unit.)
2) Some factorio products require a ridiculous amount of resources or items. (Personal laser defense, satellite, etc.)
In reality, these take very long time to make (combat with efficiency updates and multiple threads) and they generate large amounts of nasty waste. (That could be handled as any other resource - from direct disposal to storage, recycle, burning or plasma gasification).
3) Make a small adjustment to the map generator - set up larger mines further from the initial position (using threshold for the Perlin surface as a function a/(b+x^2+y^2)+c).
4) Improve belt logistics: Line terminator, internal/external line swap, inserter target line. You can do these things already by some very silly workarounds that take too much space. In reality these are straight forward adjustments. By improving belt logistics you can increase the number of inputs, satisfying the level of complexity.
5) Circular radar with evolving (and actually upgradable) resolution and range.
6) Three shades of sea with the top one accessible by foot.
There should be a lot of options for survival:
7) What would you do, if you couldn't get enough coal? Some semi-manual harvester driving in circles, chopping down trees?
8) What if you couldn't get to crude oil? Get the sulfur from coal.
9) Make single rocks mineable by hand, get that much needed stone.
10) What if your base is overwhelmed? Hop in a wooden boat and swim the hell outta there! Try your luck elsewhere.

11) Upgrades are essential but the way they are set seems like a cop out. What if the tree of upgrades included something specific: Thyristors for pulse-width modulation, multiphase electricity, energy and heat recuperation, new acumulator technology, insulation, hydraulics, laser optic measurement, ultrasound, roentgen, cryogenics, superconductivity, nanotechnology, graphene, nanotubes. These are not only fancy names, but they can also wake interest with basic explanation of these technologies.
12) Large rocket parts, cars, factories, etc. should be made by large factories. Use rails and wide flatbed carriages instead of belts.
13) Logic circuitry - I think this was a bad choice from the start. You already have mass produced tiny processing units versus these huge single bit gates as giant machines. Use a two/three/multi input machine with the power suit concept instead. Put all gates and connections inside it.
14) Introduce better vehical physics (two axle). Build a heavy truck with a carriage for mining deep in enemy territory or with any number of useful weapons.
15) What about some reference on The Begum's Fortune (Ocelové Město) by Jules Verne. Now I understand that everything that's rotating requires a large amount of textures, the bigger the worse. I guess anybody would love to see large canons riding on rails, firing CO2 (or other chemical warfare) or nuclear projectiles. (Upshot-Knothole: Grable)
16) Steam engines should work only with T > 100°C (pressurised water). Use large flywheels to reduce outages.
17) We desparately need a true power station: Boiler/reactor, (heat exchanger), turbo generator (two-stage with reheating), cooling tower.

Thank you and best of luck

[Kayser]

I think some of these are quite interesting. However, we will have a hard time discussing these if they are all in the same thread. If you picked your top five ranked on a gameplay value divided by complexity rank and elaborated on each in a separate thread I think it will be easier to flesh out some of these.

[FishSandwich]

However, we will have a hard time discussing these if they are all in the same thread.

This. There's a reason the suggestion rules state to put one suggestion per thread.

[Furan]

Of course. I made only a shallow research of the huge amount of ideas presented on this forum.
Soon I will elaborate on the radar idea. Something more specific has just crystalized in my brain.

[bobingabout]

1) You can't make an electric engine out of a combustion engine. (As it does not seem to be a generator/motor unit.)

I agree with this one, there should be an engine, and a motor. Upgrading an engine into an electric engine is nonsensical. A real motor would be made out of copper wire wound around a silicon-iron core with magnets (or copper wound coils for an electromagnet) in an iron shell. there are other parts to it too, but a logical recipe for it in the game could be to use several copper cables, a copper plate, a couple of iron plates, and a couple of steel beams. You could even introduce a new intermediate, an electromagnetic coil made from a steel plate and a copper cable, and use these as a component for the motor. Maybe I'll do this in my mods.

8) What if you couldn't get to crude oil? Get the sulfur from coal.

I think turning petroleum gas into sulphur is dumb myself, sulphur is a contaminant that is removed from the precious fuel gas. I'm thinking of a mod module that redefines the entire oil processing tree.

9) Make single rocks mineable by hand, get that much needed stone.

I think this has been done by a mod before. It is a good idea.

13) Logic circuitry - I think this was a bad choice from the start. You already have mass produced tiny processing units versus these huge single bit gates as giant machines. Use a two/three/multi input machine with the power suit concept instead. Put all gates and connections inside it.

As an electronic engineer, the first thing I think about after reading this is PAL/GAL chips... Programable Array Logic. If we have a block in the world with wires connected to it, why not have a 10x10 grid where we can choose connections and wires to design our own logic circuits within one entity? It makes perfect sense to me.

fancy

16) Steam engines should work only with T > 100°C (pressurised water). Use large flywheels to reduce outages.

Another one that has been greatly discussed over time. Generators should not be powered by hot water, but instead steam. Boilers should take water in, and give steam out. More boilers means more steam. If you are famillier with the recent changes to solar panels made for the 0.12 release, you'd know that all solar panels function as a single entity to cut down on calculations. A group of boilers next to each other could also count as a single entity with steam production multiplied by boiler numbers. More boilers means more steam produced faster. This would also mean that generators can't function as a liquid dump entity, because they'll only function on steam, not hot oil, or hot sulfuric acid.
If the fuel source liquid was defined in a lua tag, this would also open up other modded liquid to electricity options.

[Roodvlees]

What do you think about the Sea Block mod pack? Especially the metal smelting is more complicated with a lot more metals.
I also love the new buildings, of many different sized.