Frontloading could work, as long as there are restrictions on the multiclassing system. Some prefer a more organic approach to their character gaining levels though -- see how D&D 4E's Essentials classes generally gained new class features as their levels increased -- and I suppose that could also help limit the class-dipping that could be done.
Taking ideas from Star Wars: Saga Edition
(D&D 4E's predecessor, which is effectively a 3.5/4 hybrid) and 4E in general, here's how I'd envision the whole thing...
1. When creating a character, you must first determine your stats. You could
-> roll 4d6 (drop lowest) and allot the results to each stat
-> use an array and allot them to your stats
-> use a point-buy system, alloting your stats
2. After determining your stats, choose your race. Race will give you a
-> minor boost to one or two stats
-> racial ability (for multiple racial abilities, e.g. genasi, choose one)
[ I know they announced the re-introduction of racial penalties, but I'd rather not return to that since it's outright redundant. The strongest halfling can never be as strong as the strongest human in 4E, why add a penalty to the halfling's strength on top of the inability to boost their strength in the first place? Because of lore, that is mutable anyway? ]
3. After determining your race, choose your class. Class will give you
-> generic benefits, with special multiclassing restrictions at level 1, like
** if level 1 normally gives you triple the maxHP of your normal class (e.g. Fighter d10 = 30 HP at level 1), multiclassing to a different class does not give you that HP (Wizard 1 [18 HP] MC Fighter only gets you 18 + 1d10 HP, not 18+30 = 48 HP)
** if level 1 has a whole slew of features, multiclassing to a different class lets you gain only a specific set (or one of a specific set of class features, if not a weaker version of class features)
-> these generic benefits apply regardless of build, background or theme (see below). So a level 3 Rogue with the Brutal Scoundrel build who takes 5 levels of Fighter gains the same benefits as any other level 5 Fighter.
4. After determining your class, choose your build. Build gives you
-> specific benefits, similar to that of a "role" of sorts. These are special class features that cannot be acquired via multiclassing.
-> these progress independent of your class, as this is based on your character level. This means that the abilities acquired here would have to be compatible with other classes whenever possible.
5. After determining your build, choose your theme. Theme gives you
-> skills or subskills that help you be better at certain instances.
-> when leveling, you can either improve existing skills/subskills, or acquire new skills as you go along.
6. After determining your theme, choose your background. Background gives you a minor bonus, typically a skill training or bonus that normally isn't granted by your theme, or a language that your race would normally not be able to grant you, or what not.
It's a lot
more complicated at first glance, but given the modularity of the whole thing, you can basically stick to step 1 and be done with your character, or you can progress to step 2 and 3, and stop there. Or, you can get all of them except for #4, or you can ignore #5 completely.
Whenever you level up, you'd acquire the following...
1. Class level. You can choose to gain a new level in your own class (which would have new class features) or you can choose to take a different class.
2. Build feature. At certain character levels you gain the benefit of your build. This would show that no matter how much you dedicate yourself to a new field, there will always be some way your original class would influence you (again this can be discarded by those who feel that it breaks their sense of versimilitude).
3. Skill points. Unlike in previous editions, skill progression would be very slow, and basically would amount to as follows:
-> 0 (untrained). Can do trivial tasks effortlessly, can handle tasks that apprentices can do with some capability (depending on natural aptitude). Cannot do tasks that require training.
-> 1 (apprentice). Can do apprentice tasks automatically, although anything that requires high amount of skill would be anywhere from challenging to impossible (cannot perform Master tasks, journeyman moderate DC, expert difficult DC).
-> 2 (journeyman). Journeyman tasks are easy DC, expert tasks are moderate DC, master tasks are difficult DC, Grandmaster tasks are impossible. Gain 1 specialization [sub-skill] feature.
-> 3 (expert). Auto-succeed journeyman tasks, expert tasks are easy DC, master tasks are difficult DC, grandmaster tasks are impossible. Either gain an additional specialization, or enhance existing specialization.
-> 4 (master). Auto-succeed expert tasks, master tasks are moderate DC, grandmaster tasks are difficult DC. Either gain an additional specialization, or enhance existing specialization.
-> 5 (grandmaster). Auto-succeed expert tasks, master tasks are easy DC, grandmaster tasks are moderate DC. Either gain two additional specializations, or enhance two existing specializations.
Specializations would be special abilities within skills that in some ways can be considered as powers. For example, a Thievery journeyman can acquire the Locksmith specialization that allows him greater proficiency in creating and unlocking locks, while an Athletics expert could have Climbing specialization that allows him better climbing abilities, including the ability to help others climb better when he's the first among them. An Arcana expert could be specialized in Improvised Spellcasting or Spellcrafting, or what not.
To recap, this is how I'd like it to happen:
1. Class levels provide HP, base features and other stuff that can be acquired via multiclassing
2. Multiclassing would be restricted to minimize the need for "class dipping"
3. Builds would further make a character unique, by giving him what you could consider as "character features based off his original class"
4. Themes would further make a character unique, by allowing him skill sets that are most compatible with character concept, instead of class concept
5. Background would effectively seal the deal, by allowing a minor bonus based off the character's history.
I suppose you could translate "builds" in this scenario as 4E's "themes", whereas the idea for multiclassing and single classes would be more reflective of 3.5 and SW:SE