What I've done the past two games is put the sovereign on path of the mage and ignore all other champions. On path of the mage you can get abilities that will really make a huge difference, and you can get them pretty quickly. Evoker + Savant + Fireball = a hero who can really turn the tide of battle. That's what a Champion should be.
Path of the warrior and path of the assassin are not worth pursuing anymore. Defender is only marginally useful. Except for a mage with very useful spells, I would happily remove any champion from an army and replace it with a trained unit. And the trained unit will do more damage, have more hit points, better defense, and move faster (charge anyone!).
Why can any trained unit at level1 be designed with the charge trait, but with champions only a high level assassin can do that?
No champion can do as much damage as a trained unit unless he has one of a handful of rare weapons which you might never see in any given game (lightning bow, heart seeker, scythe of the void, etc). Why?
With some metal and labor I can equip a whole squad of trained units with chainmail, but I'd have to spend 800 gildar to equip one champion with chainmail. Why?
It used to be the main reason I'd level up a warrior or assassin was to get the special attacks. Now every regular unit can have a special attack, and the ones that only champions can get take a lot of levels to get to. And correct me if I'm wrong, but did Vital Strike get nerfed in FE? With the semi-random card shuffle of powers in the past you could get lucky and get crushing blow and stun early on and then you are very dangerous. Now any trained unit can have crushing blow, and stun is way at the end of a long branch of powers that are only marginally beneficial and I probably wouldn't have picked except that they lead to stun.
And I hate the XP splitting thing. There are only so many things to kill, and since I have to split the XP for defeating them I've been leveling up one or two champions and ignoring the rest. Typically I've take a mage + warlock for the sovereign and then my first other Champion becomes a defender, I'll pick the healer trait early on so the defender helps keep the army moving along and killing things. If they didn't have the healer trait, I might not even keep the defender around. And everybody else is less useful than a squad of spearmen so they go park in a town and rot there so they don't take any XP away from the other two guys.
One thing I do like is the fame thing, where you get a level appropriate champion once your fame reaches a certain threshold. It's awesome that I get a level 10 champion for free...too bad he's not as good as a militia squad.
My suggestions:
1) More special combat abilities for champions
2) Better / More useful special combat abilities
3) Fewer steps through the trait tree to reach special combat abilities. (seriously...why is stun waaaaay at the end of a line of stuff I'd never otherwise want?)
4) If you give me a level10 champion....let ME pick the 10 traits he has.
5) Undo the XP split, or otherwise make it worthwhile to use more champions.
6) Better access to good items. Maybe a special NPC shop where you can buy a few cool things? Maybe higher chance of cool things popping up randomly? Or how about item creation as in MoM? Spend 1000 mana to get an axe with extra damage and some big fat fire damage.
If we can't do some or all of the above, then you might as well get rid of the champions and just keep the sovereign. ....and you might as well get rid of all the paths except mage.