Both forms of leveling in the KR games are fun in their own way. KR makes encounters between your hero and enemies feel more important, bcause the hero in turn gets insanely strong. Frontiers and Origins give a great feeling of permanence amd satisfaction as the hero becomes stronger for good. However, even though the hero is a full strength and no longer has the worry of constantly fighting to get more powerful, we don't get the pleasure of working towards that strength again, since restarting the game is not worth it. So, I propose that there be an option to play the game in which the KR and Frontiers leveling systems be combined.
I will use Arivan as an example, because he's the best hero that all players with Origins have. The player must 1st level up Arivan normally, acquiring all of his stats and powers. Once he reaches level 10, he is at a lower or equal level statistically at the start of in-game battles (if this optional method is activated). All 36 of his skill points can then be set to be leveled up for each time he levels up in-game. What this means is that when Arivan is in an actual level, he can gain more experience from killing enemies. He levels up faster and has reduced stat growth, but when he levels up, he gets 4 skill points worth of abilites added for him.
To put more emphasis on this, I'll go into detail. He will begin each battle with stats close to or equal to his maximum so that the game isn't impossibly hard at the beginning, but have none of his skills. Upon reaching, say, level 11(if his starting level was still 10), he then acquired the skills the player set for him to acquire; let's say Seal of Fire II for instance. He now has the skill at the 2nd level of power, and sightly higher stats than at level 10. Stat growth can be smaller, but remain significant. As he burns Gnolls to ashes, Arivan gets to level 12, with more stats and the purchase of his tornado skill, also costing all 4 of points. Later, he eventually reaches level 13, where his stats increase again and the player decided to get him Lightning Strike II, which cost 3 points. The 1 extra point was saved. When he gets to 14, the player uses the 5 points to get his 2nd form of the tornado, costing all 5 points. This continues until level 20, where he obtains all of his skills at maximum power and has about 50% more stats. He is stronger, but only if the player used him often enough to actually reach that potential, which usually requires an Elite Stage or the final mission. This also comes at the risk of losing lives at the start since he has no abilities to use, forcing the player to use their spells and towers very carefully.
Having this would give players both the finality of leveling up a hero, and the constant satisfaction of killing enemies to get more experience. It'd add more tactics to the gameplay and make heroes more fun to use, as well as altering the playstyle in each level, drastically at higher difficulties. I really do think it would give players a better connection to their heroes, forcing them to use their strengths and weaknesses to their advantage, and fully utilize the differences in ability costs as they carefully pick and change, outside of battle, how the hero levels up, allowing for more experimentation.