This is probably a difficult question, but I will give a try anyways:

When I select an instrument in Ableton (either from the sound bank or plugins like Serum, Spire etc), they often tend so sound a little flat compered to professionally produced songs. I understand that these artists have the best equipment, good sound engineers and often great producing skills themselves, which will make the answer complicated I guess.

But are there some "simple" ways to make an instrument more wide and powerful? I have tried panning each instrument slightly to the left/right and experimenting with the EQs a bit, but I'm still not quite happy with the result. I'm far from a pro in Ableton, so I'm sure there are better ways to solve this.

Hope I can get some good tips here

Maybe a little reverb?

THAT is a very broad question!
Depends on the type of sounds, its timbre. What flat does mean? I'm not sure comparing presets with songs is very meaningful, the context isn't the same at all...

You could need some delay and reverb to get a sound "wider"; you could need to extend its timbre, frequency register or layer different sounds to get something fuller... or wider too.
Some phase shifting could be a solution; or some gentle chorus/phaser etc could be what you're after. Saturation can help to sound fuller, eq'ing too, panning, compressing...
Making other sounds "thinner" to contrast your "main" sound is probably a technic used in one of your reference song.

Maybe you should be more specific about the kind of sound you're after.

You're right, it's a broad question. I have just noticed when I choose an instrument (like a lead synth) that I've heard in other songs, it doesn't sound as powerful in my Ableton files.

But I will try some of the tips you gave here, and see how it goes

Some basic plugins that will help
- Reverb
- Chorus
- Delay

As mentioned, really depends on the sound and the desired effect.

Other ways to make things sound more full and powerful depending on the synth is if there are multiple oscillators you can detune one slightly then hard pan L/R each osc. In some cases, duplicating your synth track and doing the same thing, detune or slightly change one of the tracks and hard pan each one.

Another method using double tracks is to slightly delay one from the other by just a few mS. Think of how a vocal track chorus section sounds so full. Often it is because they use multiple layered takes that are all slightly different and panned different.

Experimentation is always a good thing here as you will learn a lot.

More often than not tho, with instruments like Serum you just need to play around a bit and find the controls you need to make the sound occupy the stereo field. Some presets you select may be specifically for mono playback