Should pure classes get more than they have now? Yes. Should that be accomplished by taking away things that multiclasses get now? No.
The challenge, once again, is for Turbine to make all enhancements useful - not just some, they seem to be already aware of this, which is good.
Pure classes need, and needed some kind of boost. No arguments. Capstones was one way to get partway there, but is a small and arbitrary way as well - level 20 and 18 gives something but level 19,17,16,15,14 does not for example. So part of the answer is that ALL levels of a class, or at least every other level, gives something that benefits that class other than 4-10 more hp.
Casters get more spells, so casters are in a different category than melee. Most melee classes dont do more damage adding one more level - while casters do. A level 12 fighter and a level 20 fighter can do about the same amount of damage, and if you add 6 levels of a 'better' tier 1 from another class - do MORE damage adding in something else than taking Kensai 3 for example. Any mix and match of melee tends to give some kind of benefit - in part because right now a lot of tier 1 lines or splash bonuses are front loaded. (Similarly, a level 12 Rogue/8 fighter and a level 20 Rogue disarm traps all the same)
Currently a tier 3 line requiring 18 is just an artificial number tacked onto the flawed D&D system TSR and then WotC made from edition 3.0 and onward. While Turbine has done a lot to try and work within that system to add some better balance (and sometimes failing...) - this new system could possibly do a great job in fixing a decades old problem.
But for that to work, all levels, and all higher levels of enhancements have to give increasing benefits to staying pure - which currently they do not for many cases. Haste boost as one example. Level 1, 1 point. 15% increase. Level 4? 4 points, a 5% increase. What is wrong with that picture? Every line of enhancements is front loaded in BOTH cost AND effectiveness, which is very wrong.
Every point of AP should be as important as every other point - if 1 point gets you 15%, then 4 points should get you 60%. While that will be changing, most people complaining seem to keep missing that simple but critical fact.
The fact that so many early bonuses are big, and later bonuses get smaller is why so many multiclasses are out there now. For a melee is almost always better to multi than go pure - that 1 or 2 extra feats for a fighter cant compare to Tempest 1, DoS, FB 1 or whatever - in part because feats are also useless other than the few major ones that everyone takes. (On a side note, I have mentioned Pathfinder before - because they solved this problem already - every 2 class levels you get something. Level 20 you get something bigger. Every class, not just casters - it works well)
However saying that pure classes need more power - does not, and should not, require actively taking AWAY things from multiclasses that they have now. Which is what the 3 tree limit and only accessing 1/3 of class enhancements per tree would do. No rational person can honestly say losing 66% of what you had from yesterday to today is not a nurf.
The most sensible solution should be pretty clear;
- Tier 1 Prestige lines give SOME benefit, but not a ton - but every tier 1 should be useful in some way.
- Enhancement lines get streamlined - so 1/1/1/1 (1/1?) not 2/4/6, or 1/2/3/4 - which is already going to happen as stated.
- Higher tier Prestige lines give a greater benefit than simply +1 better than the tier before. This will encourage all on its own specializing over multiclassing.
- but also - higher level enhancements BY THEMSELVES get better - so even if you do not put points into a prestige line - the points you DO spend matter, and make you better. A fighter should not HAVE to have two prestige lines to be good - if one capstone costs 41 points - the other 39 points they spend on just fighter/race enhancements should by themselves be good enough that they dont feel ripped off for not having dual tier 3 prestige lines.
- Capstones would be like a tier 4 - better again by at least 1 full step over tier 3 - and you only ever get one of them per character no matter what.
- Tier 3 should also be only ever one per character no matter what.
- Class Enhancements should not be limited to specific trees unless very specifically that enhancement is clearly only appropriate for one line. there should be very, very few enhancements that count like that - unless say you also get prestige specific enhancements that unlock once you take a prestige line.
- Racial lines should not give tier 3 access all by themselves - this has been already recognized by Turbine, but most people arguing here are arguing as if racial will still have a full prestige line unlock.
- No limits on the 'amount of trees' you can access - the amount of points alone will be a big enough limit to 'superpowers' that some seem to fear. Combined with bigger bonuses for later tiers of lines - and even if a level 12 in a class gets access to tier 3 because of relaxing level restrictions by 6 points say - a pure class is still going to have access to more, and higher, and more powerful enhancements than a multi.
Given the above, there is zero reason to ALSO limit multiclass builds by forcing a limit of 3 trees, AND locking out every other enhancement from other trees once one tree is picked. That is both overkill, unnecessary, and quite simply punitive, and not balancing at all. And quite simply seems to be an argument for fitting things on one page of a UI - not what is good for the game.
People all up in arms and quivering in fear of the 'superman' multiclass builds that might be possible are completely aiming for the wrong target.
Make taking 2 more fighter levels just as good as taking 2 more caster levels is to a caster - and the problem completely goes away. You can still choose to multiclass for versatility - but you dont lose access to the lower level enhancements as people want to make happen now.