No Rogue Assassin/Thief?

[DwarfFighter]

From what I can tell, you can readily combine classes with multiclassing, but is there a way to combine multiple archetypes within the same class?

[NecessaryWeevil]

No, unfortunately not.

[Amnestic]

Officially, no, however there is space for DM flexibility. The downtime training rules offer this:


Training

You can spend time between adventures learning a new language or training with a set of tools. Your GM might allow additional training options. First, you must find an instructor willing to teach you. The GM determines how long it takes, and whether one or more ability checks are required. The training lasts for 250 days and costs 1 gp per day. After you spend the requisite amount of time and money, you learn the new language or gain proficiency with the new tool.

Usually people point this towards getting another skill proficiency, but it could be stretched to subclass features too. I'd probably change the cost and time spent training if they did go down that route but I could certainly see dipping into different archetypes as a valid use of downtime personally. Worth discussing with your DM/group to see if they'd be open to it.

[Man_Over_Game]

I tried to make a specialized feat that counts as taking 4 "specialization levels" into a class that you have more class levels into, and specialization levels only grant subclass features that you don't already have.

So, for example, a level 6 Fighter can take a feat to gain 4 Fighter Specialization levels to count towards a Fighter subclass of his choice. He could do it again into Fighter, so long as he has more than 8 Fighter levels the next time he takes that feat. You can push these specialization levels towards a subclass you already focused on with this feat, so you could be a level 20 Battlemaster with level 12 Samurai features or something by using 3 feats, or have 4 Arcane Archer, Samurai, and Eldritch Knight, or any combination thereof.