You're looking at the wrong things, similar to how developers go astray. When you look at what matters, CoD 1 is similar to modern CoD games.
The core hooks of a game are really what builds and keeps a fanbase. The hooks of Call of Duty remain the same. CoD was a PC game that made the transition to console, which is why it took a few iterations to gain traction (I actually dropped Halo 2 for CoD 2), not because it drastically changed it's formula. As CoD progressed, it did one thing fundamentally different than Halo did: it enhanced it's hooks while Halo obfuscated.
The perk system and create-a-class are enhancements to what was already a class based shooter. The hooks of CoD where it's fast gunplay and class-based "realistic" warfare. None of that has changed.
Now look at Halo. The hooks of Halo were that it was a game of skill that pitted opponents against each other on equal footing. Players fought to gain temporary advantages around the map (powerups, power weapons, position). Movement was crisp, shooting was skillful and the game was predictable. EVERY single one of those hooks has changed. Thats why Halo is in decline.
When you look at "what matters," Halo 1 is similar to modern Halo games.
You're just arbitrarily downplaying the changes to Call of Duty as nonexistent or inconsequential. Call of Duty hasn't changed a lot? Come on, now. Compare footage from CoD 1's base gameplay and that in Black Ops 2. The newest Halo title and the newest Call of Duty title have both clearly added a ton of new ingredients since their first iterations. Thankfully, in both games, m
ost of those silly spices can be removed with custom gametypes.
Besides, both franchises are still highly recognizable by their gameplay. You act like you can't even tell Halo CE and Halo 4 are a part of the same franchise.
It's extremely subjective to say the perk system and create a class systems are definitively enhancements when assessing Call of Duty's competitive value. I would argue that games like Team Fortress 2 do (or did... until all the new weapons came out) class-based warfare "better" because each class' equipment is consistent no matter who is playing. The weapons and abilities tend to be more balanced as a result, due largely in part to simplicity.
If one of the hooks of Call of Duty is its "realistic" warfare, then again I point to Call of Duty 1's multiplayer as more representative of realistic combat in the sense that you are not running around calling in air strikes from laptops that do not appear on most character models. You are not hiding in a corner and controlling aircraft or predator missiles in the middle of a firefight. You do not have special perks or unique weapon attachments. The change from WWII to modern warfare was largely a justification for the introduction of those sorts of features. It was also due to a generation of gamers growing increasingly tired of the WWII shooter, a sentiment that had its roots even in the PS1 generation. Sort of like science fiction shooters have fallen by the wayside a little bit since modern warfare titles became the rage.
Your gripes with balance changes don't negate my point. Compared to old school CoD, Call of Duty Black Ops 2 has a more random spawn system, more unbalanced and cluttered maps, perks (random, specific to each player), kill streaks (random, specific to each player). The gunplay isn't as prime as it once was, either, with continuously decreasing recoil, more emphasis on automatic weapons with inconsistent spray patterns, and nuttier hitboxes.
And seriously, all this because some posters (not you, specifically) can't respond to a hypothetical question that really doesn't require a position on any of these arguments to answer.
Will the next big shooter title in eSports be a new IP or one that already exists (like Halo once was, like Call of Duty is)?
I, for example, would say it'll probably be a brand new IP, because I can't really think of any under the radar, up-and-coming shooter franchises on consoles that have sufficiently inventive and refreshing base gameplay to offer anything different.