There's a bunch of different ways to go, certainly.
I don't think that the Dragon Knight will be a playable character in Divinity 3 - the top down viewpoint Larian said Divinity 3 will be using suggests that it will be purely ground-based fighting, and I don't see the point in bringing in a playable Dragon Knight if he/she can only fight in human form.
Dragon Commander will probably have to do as the Dragon-form based game.
The actual numerical value of the starting level is irrelevant. At the end of Divine Divinity your character is (generally) pretty much untouchable, with only a couple of the end bosses a possible threat.
Look at Laiken. He's been ruling Maxos' Battle Tower for - in his own words - hundreds of years. In story, he's a super-powerful necromancer. In game? He's level 18, the same level the Dragon Slayer was after a couple days spent killing goblins and bandits.
IMO it would be a little lame to simply drop the Divine One's abilities until he just happens to be a match for the most common generic opponents at the start of Divinity 3. That wouldn't really bother me (character resets being a long tradition in RPG sequels), but from a design perspective I think it would be better to play a different character.
They might not have to - A level 20 starting hero could be used to tear through huge numbers of level 5 goblins, for example - that would give the player a sense of power. (You wouldn't get much XP or gold from the quest, though.)
Increasing the skill caps to, say, 30 points (and keeping the Divinity 2 level requirement system for higher ranks in a skill) would also make handing the player 20 skill points at the start of the game less game-breaking.
Far less game-breaking than Flames of Vengeance, where the skill caps were increased by 2 points to 15, and the game handed you about 40 skill points. You literally ran out of useful places to put the skill points.
Like i said befor, even though the dragon knight had no special connections with the divine i belive they could give the dragon knight even more of a story with helping the divine get to his goal.
Hell to be honest something i think would be another cool thing is a 2 person story as in you start out as a new member to the divinity story and go though what ever is needed story wise but later on you meet the dragon knight and from then on out when ever is needed or i guess wanted you could switch in between the 2 each haveing there own ups and downs of course, seeing how it would not make much use of the new guy, you could only play as the dragon knight as a dragon and fight in the air,then fight on land as the new hero.
That's an idea as well - but substitute the Divine for the Dragon Knight (for reasons I said above). Alternating chapters between the Divine and New Guy would satisfy both the character creation of a new guy and the needs of ending the story with the Divine.
However, it might be a bit disruptive for players, and it would be hard to design, you'd need to keep the two characters separated because of the power gap.
