A theme that gives an extra cantrip would be nice.
But no matter what, themes could now easily be used for multiclassing:
Theme: Multiclass
Level 1 benefit: A class specific multiclass feat.
Level 4 benefit: 1st power swap feat
Level 8 benefit: 2nd power swap feat
Level 10 benefit: 3rd power swap feat
(note that the wording was: MOST themes offer an additional benefit at level 5 and 10!)
As alternative, you could require a class specific multiclass feat as prerequisite and make the first level bonus an at-will of a class as encounter power...
I would do, at 1st level an at will of the class as an encounter power, and 1 imp or weap prof of the class
at 5th level 1 1st level class feature
at 10th level 1 1st level class fearure
and can take utlity powers from the class...
[sblock=examples]
Fighter: gain a weapon prof, and 1 fighter at will as an encounter power
at 5th level your opp attacks stop movement
at 10th level you mark anyone you attack
cool, but without the punishment for the mark, and the wis bonus to opp attacks, still less then a full fighter
Wizard: gain imp prof and 1 wiz at will as an enconter
at 5th level gain 2 cantrips
at 10th level gain implment mastery
cool but wizards spell book is still out of reatch, and you need a feat to be half the rituel caster
rouge: gain weapon prof and 1 rouge at will as an encounter
at 5th level gain 1d6 sneak attack (does not increase with teir)
at 10th level gain rouge weapon talent
cool but not over the top...well except maybe for a twin strikeing dagger master ranger
Cleric: gain holy symbol prof, and 1 cleric at will as an encounter
at 5th level gain 1 channel divinity
at 10th level gain a 1/encounter healing word as half level
[/sblock]