Sorry, this got revived and I didn't notice right away. Let me try and respond to some of the concerns.
This is, imho, a horrible idea. Even if the pcs don't cherrypick, it throws off any chance of balancing encounters without a nightmarish amount of work. And how do you figure xps if you're the dm? Does a 7/5/5 fighter/barb/druid earn xp as a 7th level character? If he defeats a fighter 8/barb 6 how many xp is he worth? What CR is he? Is he the same, higher or lower CR than a fighter 8/barb 5/ranger 5? Than a fighter 9?
Well, we all realize that CR is still 'relative'. Given a certain party make-up, and certain terrain, you can pick 3 encounters with the same CR, and one will be easy, one medium, and one deadly. *ALL* encounters need to be somewhat watched for appropiateness.
As for your questions, a 7/5/5 would be about as powerful as a 17th level character;but with fewer HP, and lower skills, and less magic. So, like anything else in D&D, you estimate it; probably around 14ish. With the above mentioned variability, it shouldn't be *that* big of a deal. Now, he may be hanging out with 10th level characters (about same XP), but you figure out the party level that way. NOTE: since it is easier to multiclass, people may also feel less 'forced' to min/max the choices; and may lead to combinations you would not get if you had to pay 'full price'.
Worse yet, what the hell do you do with a fighter/wizard 5/5? Is that a 5th level character? How do you figure xp for them, or award xp for defeating them?
I do not mean to sound flippant, but is 3E/D20 the only role playing game you have experience with? You seem very nervous with DM flexibility. If you want a 'rule of thumb' it seems like adding 2/3 of the other levels would give a decent approximation. Since I believe this will only be used for characters, the problems for 'monsters' isn't really a problem. Plus, we tend to not play with the FR tendency of Star Wars races syndrome.
As for the 5/5, it depends. His 'CR' is probably about 8, but his level for skills, HP, spell effects, fear,etc is only 5. (For instance, a ftr/wiz 4/4 would still be susceptible to Sleep spells.)
It sounds like, under his system, it's impossible to achieve a balanced multiclass character- let's compare Bob the fighter 5/wizard 5 with his 6th-level fighter buddy Tim (equal xp):
huh? It takes 15,000XP to be a 6th level fighter
It takes 10K to be 5th level, so the other 5K could be used for two levels of Wiz. (almost 3 -needs 1K more)
So try the comparison again, with a ftr 6 and a ftr/wiz 5/2. The second one is still more powerful, but not by as much
Remember, the ftr 6 will have 1D10-Conbonus more HP, skills could be at 9 instead of 8, and TWO extra feats (for 6th level and fighter bonus) Is also one step closer to the ability bonus at level 8
The dual gets to cast a couple of 1st level spells, and gets some extra skill points. Also gets two defined feats.
As for stacking, as of now, almost everything stacks, except for HP and levels. This is the first time trying this, and some changes will likely occur. I already have to make some decisions. I was Rog1, and was going to go up a level, Do I go Rng1? or Rog2? I really wanted the benefits from rng1, but I really couldn't afford to gain no HP this level (most likely).
Now, is Rng 1 going to be enough to lure me from gaining HP and another D6 in SA?
Where I think this will really break is in the lvl10-lvl14 area, where you can start picking up 4-5 levels in something instead of one level in your main.
I am powergaming this a bit, but mostly because the other characters are pretty weak and we need someone that can deal some decent damage. While I think the tweak is not balanced, I really don't see the impending doom that everyone else is forcasting. It will be interesting to see how this turns out in a few more levels.
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