Normally in these games a unit ranking up to rank 4 is a "nice" bonus for a unit, but not a real goal. However in the current build of the mod that will be releasing soon I have been spending a lot of time working on making this a very important aspect of the Mobile Suits.
What this means is a rank 4 mobile suit will be something of a goal for some players. An example would be the Powered GM's. I find I use them as a kind of "fill in the gaps" defensive team during the 3rd timeline. Once I move on to the 4th timeline I use these now more advanced units (for some of them have gained ranks due to all the killing they have been doing) as a hit-and-run tactic. I send them into an undefended section of an enemy base, kill a few targets, then run away.
Why are they better ranked up? Well, in the original game as a unit ranks up it gains some basic level bonus's. Each unit in GDI for example when it levels up gains a bonus to fire rate, health, and reload speed. That's it. What the Powered GM gets is a major increase in speed, taking it from a fast mobile suit at rank 1 to a mobile suit that can actualy out-run most enemy retaliation strikes. For example, the only GDI unit that can actually hurt a rank 4 Powered GM that is running away is the Storm Orca. This is because both the normal orca and firehawk fire weapons that do not track their target, missing because by the time the weapon gets there the GM is already long gone. And other GDI units are just too slow to catch up. The storm orca can both keep up with it's enemy and fire the gattling guns while moving, making it the proper counter to hit-and-run army tactics.
We're working with a number of different ideas at the moment on what kind of bonus's a unit should get. This is our current thoughts.
Direct-Fire Artillery Units should become more accurate as they rank up, so a rank 4 artillery unit will always hit it's target.
Indirect-Fire Artillery Units should be able to fire faster as they rank up, so a rank 4 indirect unit will get 2 shots off in the time a rank 1 gets a single shot off.
Front line units will gain stat increases that help them keep alive as they fight. This means they will gain health, fire rate increases. They will also get a one time auto-field repair when they rank up.
Certain units will gain new weapons at rank 4.
Thats about as far as we've gotten. So yea, we'll see how it plays out. If you have any thoughts about this, feel free to say something.