I just had the thought that it would have been great to keep transfering parties from 1 to 2, to 3, etc. In 4+5 you are actually taking on the same "boss" from all the preceding games, and while getting to 250th level in any one of the games may be seem insane, just think how much harder the game would be if you had to get at least that high, from playing through the preceding games and carrying over your experience! At any rate, I'd love it even if it needed to work in its own difficulty level, apart from either Adventurer or Warrior. In other words, you could play any of the games from scratch, but when you won it would be a different ending, and a totally different game.
I haven't played 1 and 2 but from what I have heard it is way to easy to get to levels above 200 in MM 2 to make transfering them to 3 possible without having the chance of players that will get no challenge at all in 3. They also planned to let you transfer characters from 3 to 4 but the late game enemies give you way to much experience and you will end up with enough xp for a level around at least 130 without grinding in 3 and this is a general problem of M 3-5. They have a well thought out system untill level 20. There you get access to Implosion and Starburst which kills all but the strongest enemies in the game and when you reach them your fighters will have enough attacks to deal up to 3000 damage in one turn. The earlier fixed damage spells are also quite broken because they allow you to takle dungeons not made for your level and in MM3 this allows you to get those power thingies early and once you get the first batch of them you have enough XP to go above level 20 which means implosion is available.