As someone with a fondness for both Tzeentchian mortals, Tzeentchian non-Thousand Sons, Thousand Sons and very offshoot/non-Thousand Sons Thousand Sons (e.g. the Crystal Harbingers), I'm quite passionate about diversifying the range.
Not diluting, per-se, but making it be a thing where there's incentive/reward/penalty for playing Thousand Sons versus Tzeentch-heavy Tzaangors versus off-piste Thousand Sons.
Frankly, I think it was a mistake to have Ahriman and Magnus be described and gamed as having 'bonded' again. Maybe allow it to be technically feasible in rules terms (model sales!- no sense ruling out options wholly), but emphasise the army-wide special rules take a hit. You can't be fielding an entirely "Magnus did nothing wrong!" army if Ahriman's there, and it can't be a "Ahriman's ambition is the only thing that matters!" army if Magnus is about, you know?
(Well, it can, moustache-twirlingly. But at least have conflicting keyword-benefits where you can only bond them by going very holy/Tzeentch's masterplan, or something.)
Anyway, the long and short of that is:
1- it's a no-brainer for sales to include possibilities like the Gaunt Summoner (DP-adjacent?) and even the Ogroid Thaumaturge.
2- you'd surely want some mortal magisters in there, with rules (strategems?) to reflect their sacrificial nature as pawns of the Astartes, but also where they can be their own force where Astartes are minimal.
3- In terms of new models, there was some disappointment that the much-mentioned 'Flux Cairn' of the Tzaangors never manifested as a terrain piece in AoS. I think GW would be missing a trick if they didn't blur some of the lines: a new Tzeentch model, but also allow a Noctlith Crown, and perhaps even 'hijacked' a Webway Gate to be chosen. (If nothing else, it's thematic and offers up further flexibility.)
4- More than anything: something new that's very specifically Thousand Sons-y, maybe a variant Astartes kit - Rubric-Revenants, partially re-animated Rubricae who've suffered immense metaphysical axiomatic damage - who's souldust's partly gone, who've been blasted by C'tan or meddled with by Ynnari or got on the wrong end of a D-Cannon - things that even the Rubricae wouldn't necessarily come out of 'intact'.
5- Sorcerous-leaning possesed - Possessed, Greater Possessed, Daemon-Engines, Hellbrutes and whatnot - have them plainly be psykers. Perhaps not fully-fledged psykers a la sorcerers, but even nominally being sorcerers gives them the authenticity of being 'real' Thousand Sons without being questonable in their proximity to Rubricae. (Like how would a non-Psyker Thousand Son come to be? If you implant Thousand Sons geneseed into a non-psychic aspirant, do they become subject to the Rubric, or do they too-quickly fall prey to the flesh change?)
6- I'd be kinda happy if they did a conversion sprue, to allow for Rubric+Sorcerer Havocs, Chaos Marines etc. It feels peculiar that they don't exist, but I can understand why they don't. (And yet, they could exist for GW but for the sale of an extra upgrade sprue...)
Obviously wish-listing, but as many others have said: it's Tzeentch. There's huge realms of possibilities to reach out into. It's sad (in a manner of speaking) that it's actually quite a tight, uniform and orderly visual army for something that really shouldn't be.
All of that again hinges on the detail: it'd be nice if there was an alluring angle that meant you could play not-quite Thousand Sons, without simply reverting to normal Chaos Astartes, or having it essentially be headcanon.
Getting the 'variety' of available options in there would be good, but in terms of new releases an Upgrade Sprue would really help add new possibilities (Havocs, etc) without totally compromising the idea that you can freely, happily play 'pure' Thousand Sons as you'd wish to.