It's not uncommon for superhero stories to have a police contact, the most famous of course being Jim Gordon. What elevates the best of those police characters is some sort personal stakes, like family ties or a shared background. Tsukauchi doesn't have that yet, but Horikoshi has already said there are several characters he hasn't got around to showcasing yet. Tsukauchi's role is primarily in relation to All Might instead of Izuku or other students, and we've only really explored All Might's chacharactin regards to OFA. With the way the story has gone, there hasn't been much of an opportunity to explore Tsukauchi's further.
Law enforcement characters aiding superheroes and even having personal stakes involved with them definitely aren't uncommon, I've seen it plenty of times and can even name a few more examples. What's uncommon is when they are explicitly mentioned to be tied to the personal life of one of the most renowned heroes of all time, know their secret identity/superpowers, and also still be in good relations with them to know about their declining health and current situation all at the same time. The difference between characters like Gordon and Tsukauchi is that we know the full scale of Gordon's relationship with Batman because the story is told from Batman's perspective, and we actually see how that relationship came about as the story is told chronologically. My Hero Academia doesn't have that luxury with Deku and how enigmatic his mentor's backstory is as he continues to get closer to All-Might in both strength and trust. That's where I'm trying to get at. His character seems minute, but his connections with All-Might and constantly reoccurring leave me thinking that there's going to be some sort of elaboration and payoff to his character in the plot. There are plenty of characters Horikoshi hasn't showcased yet as much as he would like (I remember him specifically mentioning that about Ashido in a past volume), but Tsukauchi is in a bit of a different league when it comes to how major his ties are in the overall plot and relevance in the scope of the protagonist by having an implicitly close relation to his mentor. And there's always a difference between not having the opportunity to explore a character yet and saving that type of exploration later down the line by being reliant on future story developments. A character close to All-Might's past like Tsukauchi can easily fall into the latter category, especially since his origin story and events with his mentor are still being kept under wraps. Not to mention that Shigaraki and All for One's roles in that still have to be explored as well.
Another avenue of exploration could easily come through the evolving role of the police force. As the "sympathetic" face on the force, Tsukauchi's could easily become an eye into their changes and likely an objector. He could either philosophically object to the changes or face pressure for not having a combat-oriented quirk, but either way his character would gain more from working more closely with the protagonists than from distancing himself and becoming more authoritarian. Seriously, we've seen no hint that he thinks the Noumu should be used as weapons.
Regarding the allegations that Tsukauchi's is in league with All for One, they're nonsense. AFO almost lost Shigaraki, lost a score of Noumus, and got himself beaten up and incarcerated because of the surprise attack that Tsukauchi coordinated. The idea that Tsukauchi is Black Mist is particularly dumb considering that we have literally seen Kurogiri and Tsukauchi at the same time during the bar raid. Tsukauchi was right outside as Kurogiri was attacked and incapacitated.
I can agree with him not being in league with them. I forgot that Kurogiri and Tsukauchi were near each other, I just got the idea from looking at past posts relating to Tsukauchi to see if I wasn't repeating or missing anything already said. But you have to wonder if anything is going to come out of those Noumus that were taken into custody. They aren't just random minions or constructs, but actually corrupted people who may or may not be "dead" in this state. The fact that All for One is out of the picture means that no more Noumus can be made, at least as far as we know. There could be more lurking in other secret villain lairs, especially since we still don't know where exactly they were produced, but I doubt those specific Noumus in custody are just going to be forgotten by the plot. There is some potential there for a cure to be explored down the line of the series or for either the Villain Alliance or somebody else to take control of them since they still have to be researched. I'm definitely not saying that Tsukauchi has to be the character responsible for those things (I only mentioned that as a theory I saw, one I made fun of for how ridiculously left-field it is by the way), but that those are definitely plot lines to consider for him to somehow get involved in overall. And since the notion has been established that Tsukauchi can provide a unique gaze into how law enforcement and government workings operate in this world and could possibly change, as well as the fact that we know he's been recording info on the Noumus himself, he can definitely get tied into that subplot somehow in a myriad of ways.
Tsukauchi is the most likely candidate to play a role in exploring and showing the philosophical conflict in whatever happens with superhero regulations, that's a given. But that still doesn't cover his relation to All-Might. This is purely coming from my gut, but I doubt that a writer like Horikoshi introduced that type of ambiguously major relationship that's waiting for some type of flashback to not somehow end up being relevant, rather than just having All-Might be childhood buddies with this random cop just because. He's probably going to add some type of piece to the puzzle, whether it's being some type of inspiration or support for All-Might becoming a hero or something bigger.