which is just like the battle system already implemented into the original pokemon series, but what about a party consisting of 3 or 4+ people?
Umm... since when did you think of party more than 2 people? 2 people is seem to be the best and the max to apply towards Pokemon's system. If you have played Smogon before, you would know there are plenty of strategies used during 2vs2 battle. Plus if we were going to be having more than 2, PU's team has to make a huge change to the system and remake it most of it and I think that's an overload job to do. So precisely, NO, that will not be happening in PU unless you go find a different kind of Pokemon MMO.