In fanfiction authors often using "lubimiy" or "moya lubov" for Shane's nicknames, but I think Ilya can using more interesting Russian words for Shane.
1. Солнышко - solnyshko (Sun, sunshine). A kind, smiling, pleasant person. He laughs often, smiles often, and literally warms everyone with his light.
2. Дорогой - dorogoy (Literally "darling") As in English, this is most often said to a husband or someone who is very dear to you.
3. Драгоценный - dragoceniy (darling) Same meaning as "darling," only more refined. Literally meaning "precious," as in a precious stone or a precious diamond.
4. Сладкий - sladki (sweetie) A gentle, playful name. It means a tasty person or a person who is good-looking, or simply a loved one.
5. Зайка, зайчик - zaika, zaichik (bunny)
6. Мой хороший - moi khoroshi (my lovely, literally my good). It can often be sarcastic. Outside of sarcasm, it can be said if someone has done something good, something amazing.
Cute Shane's name versions:
1. Шеня, Шаня - Shenya, Shanya
2. Шенюшка - shenushka (its look like a joke, but still cute)
3. Шанешка - shaneshka (its a russian bun with potatoes. I think Ilya could call Shane that as a joke)
1. Илюша - Ilyusha (A very tender meaning. This is how Ilya could have been called by his mother, Svetlana, Shane. It can also be used for sarcasm and anger. But I think any cute word can be used sarcastically)
2. Илюшка - Ilyushka (A more playful option. It's used either for a child or as Svetlana might call Ilya. It's affectionate, but with a little humor)
3. Илюшенька - Ilyushenka (A very soft form, probably the softest form of this name. Very tender, loving, sincere. If used with anger, it comes out very rude and unpleasant, the ultimate mockery)
4. Илюха - Ilyukha (Used exclusively by friends, most often male friends. This is not the way girl friends, relatives, or lovers address a friend, only bros)
Random words for nicknames
In the book, Ilya sometimes calls Shane strange Russian words, like газонокосилка - gazonokosilka (lawn mower). If you'd like to add some more words to your fanfic, here they are:
1. Береза - Bereza (birch)
3. Звезда - Zvezda (Star)
If you have any of your own cute or funny words, you can write me a message, I will try to translate them correctly and explain them