I think the Quranic messenger in its basic definition is anyone who passes on a message. This even applies to normal people that may pass on a message of truth. If what you utter is truth, then who does truth come from?
I think that the premise that we may need to 'look out' for a messenger has been incorrectly assumed by some from a contextual misreading of the proclamations of 'messengership' made by certain messengers in the Quran.
These proclamations in the Quran such as in 26:107, 26:125, 26:143, 26:162 and 26:178 etc has often been understood by some to mean that 'messengership' needs to be proclaimed and that they are in some way Divinely chosen and exclusively. (a possible valid criticism even against individuals like RK, Mirza Ghulam etc).
God can make use of anyone to pass on a 'message' of truth (16:2, 40:15). I have said this to you before, that can also mean you too. He may choose to make use of you to pass a convincing message to a disbeliever who may in turn accept your arguments and convert. In this capacity, you have only passed on a 'message'. Only god knows best.
Now today scripture is complete, no one can add or take anything away from it. Today's messengers can only pass on the 'truth' of the message. Please note the following verse.
023:051
"O Messengers! (Arabic: rusulu - plural). Eat of the good things and do right. Indeed, I am aware of what ye do"
Here it is clear that those who were around Prophet Muhammad were also being called 'messengers'. If we assume for a second that Abu Bakr, Umar, Uthman and Ali are real personalities (I have no reason to deny it - but we are outside scripture for now) and were active in spreading the message, then they would have been possibly included into this address.
Also messengers are not perfect clones. There were three messengers in Surah Yaseen together (36:14). Who is to say that they all agreed perfectly on every fine theological point? Humans are humans, they disagree. Are we to assume that the three messengers in this case were perfect clones of one another and agreed with everything they said to each other? Did Prophet Moses and Aaron agree with everything? (20:94) “ Clutch not my beard nor my head!). However a man operating in the capacity of a PROPHET in terms of scriptural revelation is different. In the case of Prophet Muhammad, his revelation was protected (53:3-4). However, 'Prophethood' has now ceased (33:40) and there will be no more 'Divine' scriptures or Prophets.
Furthermore, messengers also have grades (Darajatan) with God (2:253) and they too will be judged and questioned (7:6). There is also a tacit indication in the Quran, that some individuals that God wanted to elevate (possibly as messengers) failed in their duty as they became a victim of their own lusts and earthly desires out of volition.
Please read 7:175-6 very carefully in Arabic or with a good translation. When God talks about 'rafa' of a human, it usually means elevation into God's special grace. This term has been used for both Prophet Jesus 3:55 and Prophet Idris in 19:57. So 'rafa' in this context has a very significant purport. Now the same word 'rafa' has been used for this unnamed individual (or possibly a reference to a generic type of person) in verse 7:176 (If it had been Our Will, We could have 'rafa'nahu' (raised / elevated / exalted) him ...). But instead he inclined to the earth and no matter how much one would warn him he will not see the error of his ways. The parable of a dog and his panting tongue is quite apt here in this verse.
Some would argue RK fell in that trap, some will argue that Mirza Ghulam did the same. Others may have the view that Shabbir Ahmed is another. Some may say I too have suffered the same fate. My point is not to comment on anyone or myself. Each one of these personalities have their supporters and defenders. There is nothing fantastic about that. God remains the final judge.
However, it is clear, that once one falls from God's grace due to their own desires, they are no longer 'God's helpers'. God does not make use of 'misleaders' to do his true work (18:51).
Our responsibility is simple. Today we must judge any person on our best understanding of the Quranic text and accept the best meaning (39:18). 'Messages' will come to us in different ways. We shouldn't get too hung up about 'searching' for a messenger. However we should be ready to accept a better message or information when it reaches us. If a person rehearses clear verses from the Quran and we are persuaded by their strength of argument, then we should accept and mend our ways.
007:035
"O ye Children of Adam! whenever there come to you messengers from among you, rehearsing My verses (Arabic: ayati) to you, those who are righteous and mend (their lives), on them shall be no fear nor shall they grieve"
I hope that helps,
Peace.