Salamun Alaikum,
We all remain humble servants of God and to Him is our return. Let us pray sincerely that He shows, guides and keeps us on a path that is closest to the truth and His mercy. We, by ourselves are nothing. Only He can guide us.
With regards your question, the Quran does not prescribe a particular
form or
content for prayer. However, it does expect you to congregate and
bow with those that bow. So one would be obligated to follow the congregational form. This was also expected from those of the scriptures before us such as the Jews "
... and bow down your heads (Arabic: ir'ka'u) with those who bow down (Arabic: raki'een) (in worship)" (2:43)
Therefore, there would be no need to reinvent the form as one would be expected to follow the congregation pattern. All Muslims pray to God during salat, so I personally would have no problem reading my prayers behind a Sunni, Shi'a or any sect between them. I will congregate and pray to God alone.
As far as what to
say during prayer, there is no
prescribed utterance, other than an expectation that you
know what you are saying (4:43). Also, during private prayer, it is best to say your prayer in a slight audible tone
("Neither speak thy Prayer aloud, nor speak it in a low tone, but seek a middle course between") 17:110.
A lot of people make their 'duas' after the prayers when I humbly see no reason from the Quran why it cannot be said inside the prayer. After all, what is prayer for? The traditional recitations during the sitting position, like the 'darud' are also just 'duas'. They are not recitations from the Quran.
As you have highlighted the 'sitting position', I would seek to express heartfelt 'duas' (prayers) to your Lord in a language that you understand and one which resonates with you deeply. You can recite, modify and edit the traditional 'duas'. You can recite completely your own.
If you are fluent in Arabic or understand what you are saying whilst 'feel' its words', then well and good. If you do not, then I would pray in a language you understand. God made all languages and He made you. The communion with God is a
personal one and
no one has the right to dictate your personal deep relationship with God and what language that should be communed. Did Prophets Moses, Jesus and Abraham (pbut) not pray to God with their hearts in their own languages?
These 'duas' do not have to be fixed but there are many examples of beautiful 'duas' in the Quran. You can also use your own.
Please see related article [1] below for some examples.
So instead of reciting all your 'duas' outside prayer, bring them into your prayer and worship your Creator in deep meditation. He is a God truly worthy of being worshipped. Ask Him. He hears your call.
002.186 "When My servants ask thee concerning Me, I am indeed close (to them): I listen to the prayer of every suppliant when he calleth on Me: Let them also, with a will, Listen to My call, and believe in Me: That they may walk in the right way"I hope that helps, God willing.
Joseph.
[1] SOME PRAYERS FROM THE QURANhttp://quransmessage.com/articles/quranic%20prayers%20FM3.htm