This section

In this section of the forum you can propose your own army list. Please propose lists that are not present in the supplements or among the betas.
Any user can cotribute to the develop and once the discussion is over, the list can be added to the beta lists.

It is important that once a general agreement has been reached, the list is proposed again at the end of the discussion and classed as "approved by the forum". After some days I will have a look, make some adjustment if required and put online as a beta list.

Even if you don't have a list to post here it would be handy to have a list of armies that aren't covered in any of the Extra Impetus publications or wherever. For example I've not seen a list for either Saite/Rebel Egyptians or Syracusans outside the BI lists and there is certainly a gap between Early and 2nd Punic War Carthaginians. What else is missing?

Such a list might encourage people to come up with their suggestions.

Are there any guidelines (or just advice) on lists building?

no guideline, but betas are good samples.
I suggest bot to propose lists to complexes and focus on the essence of the army.