What kind of follow-on assignment would you recommend to an E-6 coming off of a broadening assignment?

As a 42A your next spot should be in an S1 shop to resharpen your skills and take over as a shop NCOIC as soon as possible. Running an S1 shop is a very perishable skill and data systems change drastically in the 3 years you'll spend on recruiting. Every 42A I have personally witnessed returning from a broadening assignment takes about a year to get reacquainted with current systems and local protocols.
After that, if you're looking to be more competitive you can apply for a challenging assignment like JSOC, USASOC, HRC analyst, or SFAB. The more common approach would be to join the AGCRA, get to know the higher positioned AG members at your location and work yourself into a job at a higher echelon like Division or Corps. There you would be able to meet higher ranking AG NCOs who could mentor you in your professional development.
Alternately, once you're comfortable in the S1 shop again, now would be the time to apply for warrant.
Hope this answers your question.

This really depends on what experience you had prior to departing for recruiting command. If all of your previous time was in an S-1, I wouldn't rush back into another S-1. If you have not worked at HRC or one of the PERCOMS in Germany or Korea you cannot go wrong with one of those assignments since you will be making personnel decisions rather than pushing paperwork forward. Additionally, those assignments allow you to interact with the most senior HR personnel. Platoon Sergeant time isn't common for a SSG and normally is an additional duty that mostly involves accountability at formations and typically often involves several sections that are too small to make up their own platoon. In those cases the Soldiers will be accountable to their section supervisor and not you for their daily work. PSG won't be a negative but unless the duty involves more than accountability it probably won't be a plus for your centralized promotion board. I would avoid specialty assignments where you are not doing traditional HR work even though most of those assignments have higher selection rate your time as a recruiter will count toward those type of assignments.

I chose a strategic command after Drill Sergeant duty and was selected for SFC while at that follow on assignment. If you haven’t hit a strategic command I would recommend that. Try and hit the Tactical, Operational, and Strategic marks, but defer to your career map because not all MOS have the same requirements and KDs