I liked the concept of the trainee units so I would like them to return, maybe in future games they could be even more expansive.
Summonners were a nice idea, their ability to summon phantoms made them unique. I found summonners to be a nifty little class, but they're not too high a priority on my list of classes in want to return in future games. If you want to be technical, you could say summonners (or summonner) returned in RD since Izuka was one.
Wyvern Knights I never particually liked, so I don't want them to return. In opinion having the use of swords (or axes in the case of the most recent games) is better than the pierce skill.
Mage Knights... I don't really know what to say about these since they're basically valkyries (FE6/7 valkyries). Maybe they could return as units who can use swords as well as magic, like they did in past games. To be honest I don't care whether these return or not, as long as they keep valkyries.
Great Knights, these were one of my favorite classes in Sacred Stones. They were like a cross between Paladins and Generals (two fantastic classes), which made them amazing tanks. They looked great also.
Rogues, this class was my favorite class introduced in Sacred Stones (from the previous two games) with the exception of the trainee classes. I know they returned in RD, but in future games I would like to see Rouges as the promoted class of thievss (yes they should promote) instead of the role going to assasins. Rogues are the 'true' promotion from theif, not assasins. A theif promoting to a rogue is the theif improving his skills as a theif, a theif promoting to an assasin is a theif changing their skills (becoming killers at the cost of them losing the ability to steal). I don't really care what happens to the assasin class.