Let us understand the Grammar behind the word “JazakAllah”
Jazaak comes from the root word jazaa' (جزاء) which according to the popular Arabic-English dictionary, Al-Mawrid, has two meanings that are completely opposite to each other.
So “JazakAllah" may either mean "may Allah reward you" or "may Allah punish you",
while “JazakAllah Khair” means "may Allah reward you with the Best / Good"
Though someone might say that the intention by saying JazakAllah is the same like JazakAllahu khayran, if it's so, then why not use the complete wordings as used by Prophet(SAW)
Masculine: Jazak Allahu Khair/Khairan
Feminine: Jazaki Allahu Khair/Khairan
Plural: Jazakum Allahu Khair/Khairan
“May Allah reward you with good in abundance" as "Katheeran" means "abundance/abundantly".
Feminine: Barakallahu Feeki
Masculine: Barakallahu Feek
Plural: Barakallahu Feekum
Which means "May Allah bless you", which is an appropriate way of thanking someone and replying to someone who's said JazakAllahu Khair to you.
As a reply you may also say:
(Below are the replies to someone who has thanked you with JazakAllahu Khair or Barakallahu Feek)
Feminine- Wa Iyyaki
Masculine- Wa Iyyak
Plural- Wa Iyyakum
They all mean, "same to you".
Feminine- Wa Anti Aydan
Masculine- Wa Anta Aydan
Plural- Wa Antum Aydan
They all mean, "you too". (Not very preferable, but it's okay to use this as a reply too).
Feminine: Jazana wa Iyaaki
Masculine: Jazana wa Iyyak
Plural: Jazana wa Iyyakum
Translation: “May Allah reward you and me too (with the same)”