Multiclass-ception: MCing into your own class (D&D 5e)

I hear people ask this on the internet, but never in my group. It sounds like an interesting thought exercise: could your D&D character "multiclass" into a subclass inside of your own class?

For example, could a champion fighter "multi-class" into a battle master fighter? Or, more interestingly, could a thief get one or two levels of assassin or arcane trickster?

Here is why - and how - I'd allow it.

First, it seems like a fair idea in terms of fluff - maybe you want your thief to gain a few arcane trickster features, like Gray Mouser, without multi-classing into wizard, or you like the four elements monk to have some aspects of the kensai.

Also, if my 2nd level fighter can become a battle master... why wouldn't my 12th level champion be able to do the same?

In terms of balance... I cannot see any problems at a first glance, although I'm sure we can find something. I'd rather forbid one or two problematic cases than ban the concept altogether, although I suspect that these rules will seldom be more powerful than simply multiclassing.

Anyway, my first instinct is to treat this as "school credits" or something similar - you do not have to take the "credits" you already have. Or like "prestige classes" that require a few levels in other classes to get to.

This is how you multiclassing work by 5e rules (emphasis mine):

Multiclassing

Multiclassing allows you to gain levels in multiple classes. Doing so lets you mix the abilities of those classes to realize a character concept that might not be reflected in one of the standard class options. 

With this rule, you have the option of gaining a level in a new class whenever you advance in level, instead of gaining a level in your current class. Your levels in all your classes are added together to determine your character level. For example, if you have three levels in wizard and two in fighter, you're a 5th-level character. 

As you advance in levels, you might primarily remain a member of your original class with just a few levels in another class, or you might change course entirely, never looking back at the class you left behind. You might even start progressing in a third or fourth class. Compared to a single-class character of the same level, you'll sacrifice some focus in exchange for versatility.

One random example:

Character Level
1	Fighter 1
2	Fighter 2
3	Fighter 3
4	Fighter 3	Rogue 1
5	Fighter 3	Rogue 2
6	Fighter 4	Rogue 2
7	Fighter 5	Rogue 2
8	Fighter 6	Rogue 2
9	Fighter 7	Rogue 2
10	Fighter 8	Rogue 2
11	Fighter 9	Rogue 2
12	Fighter 10	Rogue 2
13	Fighter 11	Rogue 2
14	Fighter 12	Rogue 2
15	Fighter 13	Rogue 2
16	Fighter 14	Rogue 2
17	Fighter 15	Rogue 2
18	Fighter 15	Rogue 3
19	Fighter 15	Rogue 3	Warlock 1
20	Fighter 15	Rogue 3	Warlock 2

It is obvious that you cannot multiclass from pure fighter to rogue 2 without going through rogue 1; you haven't complete your "credits" in rogue school until you do!

If you are a you're a "pure" fighter, you have three paths to choose from: champion, battle master or eldritch knight.

Character Level
1		Fighter 1
2		Fighter 2
3	Champion 3	BM 3	EK 3
4	Fighter 4	Fighter 4	Fighter 4
5	Fighter 5	Fighter 5	Fighter 5
6	Fighter 6	Fighter 6	Fighter 6
7	Champion 7	BM 7	EK 7
8	Fighter 8	Fighter 8	Fighter 8
9	Fighter 9	Fighter 9	Fighter 9
10	Champion 10	BM 10	EK 10
11	Fighter 11	Fighter 11	Fighter 11
12	Fighter 12	Fighter 12	Fighter 12
13	Fighter 13	Fighter 13	Fighter 13
14	Fighter 14	Fighter 14	Fighter 14
15	Champion 15	BM 15	EK 15
16	Fighter 16	Fighter 16	Fighter 16
17	Fighter 17	Fighter 17	Fighter 17
18	Champion 18	BM 18	EK 18
19	Fighter 19	Fighter 19	Fighter 19
20	Fighter 20	Fighter 20	Fighter 20

To get the 10th-level battle master features, it is not enough that you're a 9th-level fighter gaining a level - you need to have walked the whole path, and that includes having the features of a 3rd and 7th level battle master.

But - here is the house rule - once you've been through the path, you can take a step in any direction you like, including your own sub-classes. You can even change your course if you want to!

Let us some examples.

Say you're a 10th level champion.

Can you multi-class into monk or rogue? YES, if you meet the prerequisites. You start at level 1 in any of those classes.

Can you get a level three monk feature? NO, because to get that feature you need to have walked the whole path through Monk 1 and Monk 2.

Can you multi-class into battle master 3? YES - you meet all the requirements, since you'e already been through Fighter 1 and Fighter 2.

Can you multi-class into battle master 10? NO, because, even though you have all the fighter levels that are required, you do not have levels 3 and 7. On the other hand, you CAN reach you goal in the next three levels. You can even switch your path to Battle Master once you get a few more levels:

Character Level
1	Fighter 1
2	Fighter 2
3	Champion 3
4	Fighter 4
5	Fighter 5
6	Fighter 6
7	Champion 7
8	Fighter 8
9	Fighter 9
10	Champion 10
11	Champion 10	BM 3
12	Champion 10	BM 8
13	Champion 10	BM 10
14	Fighter 11	Fighter 11
15	Fighter 12	Fighter 12
16	Fighter 13	Fighter 13
17	Fighter 14	Fighter 14
18		BM 15

Notation would be something like Fighter 10 (Champion)/Fighter 1 (Battle master 3) at level 11, and  Fighter 15 (Battle master)/Fighter 3 (Champion 10) at level 18. A bit confusing, I admit, but should work well enough.

But... feats!

Yes, feats are a fair solution too. In this case, the Martial Adept feat. If we used this house rules, would the martial adept feat become obsolete? Not quite. Battle master 3 STILL requires fighter 1 and 2, so the feat would still be useful for other classes, while the champion and eldritch knight would gain a significant boost when taking maneuvers. Sounds good to me!

Would that work?

I have never tested it, but what do you think? Any pitfalls I might be missing?

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