Having given the topic a bit of a think, I have to agree with many of the earlier posters.
In order to design a 'new' rifle, questions about who the end-user is supposed to be, the role of the rifle, and the operational conditions for the rifle need to be answered.
Generally speaking, most nations use one of four calibres for their rifles. If the country would be considered a Western bloc or allied nation, then their rifles will likely be chambered in either 7.62 x 51 mm NATO, or more likely 5.56 x 45 mm NATO. OTOH if the nation is more Eastern bloc, or a current or former Soviet satellite state or client nation, then their rifles might be chambered in 5.45 x 39 mm or more likely 7.62 x 39 mm.
Unless one is planning on introducing an entirely new and improved round (and requiring entirely new munitions warstocks and magazines) then it likely would be best to plan on using one of the above calibres.
A potentially viable alternate though would be if one were interested in designing a carbine for use by police, paramilitary and special forces, or for use in engagements of 100 m or less in range. In this case, then something in a pistol calibre like 9 mm Para Bellum (9 x 19 mm), or .40 S&W, or .45 ACP. While the accuracy of these rounds at distance is typically not so good, if these pistol bullets were fired from 16" or 20" barrels instead of a 4" - 5" pistol barrel, then accuracy at distance would likely go up, as would the effective stopping power of the round.
I suppose at this point, the role of the rifle would really need to be defined, since that will at least suggest, if not outright determine which features are most useful.
-Cheers