The current ext.cap kinda upgrades will be part of the dynamic weapon mod system
Always having the same fighting style and then just adding 5% more dmg per tier is boring, each tier brings changes
The current tier 3's pretty much make up next seasons tier 10s, however theyre not really part of the normal progression. In a sense the normal tec prog stops at 9 and then gauss and APA and vindicator will be made available in a similar fashion like the current tier 3's. What I mean by that is that gausses would not really be standard issue like any tier before it, but tier 9 would still be the baseline and as the final battles progress, more and more tier 10 will get in.
Another thing to mention is that the difference in tiers is not always as big and it's not really the case that often that a next tier is just better in every single aspect. I can see certain guns staying in the meta longer than others, especially when those guns are made legendary
Another thing is that very special, unique weapons that are too fancy to have a place in the normal tec tree, will also be available through other means. They wont necessarily be better, but offer more varied and niche playstyles.
To adress your last sentence, I think that one weapons class is still the most important factor, for example you probably wouldnt see a random SMG that could fulfill an AR role better than some funky AR. Borderlands is a good example of a game where that happens sometimes where you can end up with a scoped pistol for long range and an automatic sniper rifle for crow control. This kinda stuff would be relegated to the special weapons I just mentioned above