Tiberium crystals

You could make them out of shards of uranium glass and shine a blacklight on them for a really awesome effect. Not sure if that's a good idea to put on cake though.

But this is THE thing that is C&C

There's always the main factions' logos:

Global Defense Initiative

Brotherhood of Nod

Tim Curry shouting "SPACE"

I like the tiberium approach, but I'm thinking sugar crystals with food dye. A stick of cinnamon bark could serve as the blossom tree in the middle of the tiberium field.

Alternatively, bake a Mammoth Tank cake (somehow). Probably the most iconic C&C unit:

Alternatively, the Temple of NOD:

Do a Tiberium harvester!

You could look up the flags for GDI and NOD and just make it a split design between those two, and it would be decently representative.

Unless you wanna go all out and make little edible Mammoth Tanks and Walkers and little gold and red guys fighting in a desert filled with green crystals being harvested by Harvesters.

You write "no specified edition", meaning the setting? Do you know that C&C has two main separate settings, one being the Tiberium one, while the other the Red Alert one? With multiple games for each setting.

rather than any other hex game

Well, the first thing you should know is that C&C is NOT a hex game. So if you make hexagonal terrain, whoever is wanting the cake is going to be really confused.

That's like making a Halo themed cake and asking is there anything you need to make it obvious other than zombies.

If it's not a mural of Kane, you'll soon hear the hums of stealth tanks.

Tesla coils!

There are two aesthetics: C&C versus C&C Red Alert.

• C&C uses Tiberium, factions are GDI vs Nod, iconic Nod structure is the Obelisk, iconic character is Kane.

• C&C Red Alert uses gold+gems, factions are Allies vs Soviets, iconic Soviet defensive structure is the Tesla Tower, iconic character is Tanya.

I'd say mixing aesthetics would look odd to a fan, so probably best to pick one. If you don't know which is preferred, C&C (non-red alert) is probably the safer choice.

Perhaps a simple split down the middle, with a faction on either side (NOD + GDI), and some tiberium in the middle? As if the sides were warring over the crystals.

Depending on how detailed you wanna go, you could research some iconic units and add them. Seeing Kane’s beautiful bald head on a cake would be excellent.

I'd say a very good decoration would be a black scorpion fighting a golden eagle next to a tiberium crystal.