It seems that the iMac's network card doesn't exceed 1 Gbps.

If you took a moment to actually look at the specs for that machine, you would see that it has a 1Gbps Ethernet port...

No, you can't do anything about it that.

I suppose if it's that big a deal to you, you could get a thunderbolt Ethernet 4 adapter... Thunderbolt 4 is rated unto 40 Gbps.

The Technical Specifications for the 24” M1 iMacs state quite clearly that the models with Ethernet have Gigabit Ethernet. E.g.,

iMac (24-inch, M1, 2021) - Technical Specifications - Apple Support

Thete are third-party Ethernet adapters such as the SonnetTech Solo2.5G and Solo5G that will plug into your iMac and give you a second Ethernet interface that can hit 2.5 Gigabit (minus overhead) speeds.

https://www.sonnettech.com/product/networking/ethernet-networking.html

novalisb wrote:

Is there a plan to upgrade the card in future iMacs?

Those that don’t know what Apple is planning don’t know, and those that do know what Apple is planning are exceedingly unlikely to comment publicly prior to formal announcement.

...

E' previsto un aumento della scheda nei prossimi nuovi iMac?

Aquí nadie sabe la respuesta a eso.

As for options potentially available, Mac mini and some other current Mac options do have faster Ethernet wired connections available, and Wi-Fi 6 networks (such as that used in the iMac 24” M1) can potentially be configured for speeds faster than gigabit.

novalisb wrote:

Is there a plan to upgrade the card in future iMacs?

I don't know about iMacs - but 10 Gigabit Ethernet is standard on Mac Studios, and an extra-cost option on Mac minis. Note that Apple's 10 Gigabit Ethernet interfaces can run at 1, 2.5, 5, or 10 Gigabits per second – but not at any speed less than 1 Gigabit per second.