unfortunately no word(or phrase) is same meaning with "I'll miss you" in Japanese...
cuz "Ill miss you " means "I'll feel lonely cuz u are not here" or "I wanna see you" or "I wanna feel you " or ,,,like that right??
( Im sorry , firstly Im not good at English)
I miss you nearly equal 我想你。
but we Japanese dont have that kind of expression.
so we choose those expression depending on situation.
"I'll feel lonely cuz u are not here"
anata ga inakute sabisii desu.
あながいなくてさびしいです。
"I wanna see you"
anata ni aitai desu.
あなたにあいたいです。
etc.....