Not exactly.
You can do as many sidequests as you can unlock. Seriously, go nuts. Recruit all the side characters, whatever. They're fine.
However, you cannot do any of the random battles. Further, while DLC missions can be repeated indefinitely, you will gain zero experience from the DLC missions, with the sole exception of the "Boo Camp" DLC, which is a map designed specifically for EXP acquisition (similar to EXPonential Growth from Awakening). Other rewards, such as weapons or items, are dished out normally from DLC missions, but you will get no experience. "Boo Camp" (when it releases) is thus the only way to indefinitely grind for character Exp in Conquest.
Anyway, on to other stuff:
The new Pair Up mechanics are so much better than the old ones. Choosing between the offensive or defensive setup is a very, very important choice that adds a ton of tactical depth, instead of just "Pair Up to win".
You don't get Dual Strikes if you Pair Up, but you do get pretty significant stat bonuses, even at low Support levels, and the new mechanics for the Dual Guards system are great. The more actions the pair take together, the more the system charges up, and it maxes out, it will fully block the next hit you take. If you are Paired-Up, Dual Guards will also completely negate enemy Dual Strikes. Anyway, the system is a lot more reliable, and makes you think about how and where to use your pairs (don't worry, it doesn't take too long to charge up, so you don't have to worry about it too much).
There doesn't appear to be a dedicated Jeigan, but you get several characters that serve that role during various chapters, and using them right is actually important. They can still steal experience, so you don't want to over use them (seriously, in Chapter 5 the OP character you want to rely on was only getting 1 XP per kill). But, at the same time, you can't just use your weaker units, or you will lose units. After using units like Titania and Seth, who were good the entire game, or either over-relying on Frederick during early Lunatic, or not using him at all on the lower difficulties (except once I got him to good stats in the eternal grind), it's nice to have that archetype be characters that you have to use WELL again.
The new weapon mechanics are brilliant. I don't miss durability at all (Armsthrift was a skill I prized almost as much as Galeforce). But the new changes are so big, and yet so simple, overall. Ninja stars can debuff opponents, some weapons can't counter as well, etcetera. But they all list what they do very clearly. And the new weapon triangle is such a subtle, brilliant thing. Each weapon has a background now. A color, with a small colored stripe at the bottom. It shows what class the weapon is, and what it beats. So a weapon with a Blue background, and a Red stripe below it counts as a Blue weapon, and beats Red weapons. It's obvious when you want to know, but it doesn't stand out or get in the way at all.
Overall, I'm loving this game. I get the feeling that there is SO much more depth in the combat, and I can't wait to uncover more of it.