I'm wondering if it's possible for Seraphon to have laser guns. Spoiler: Reasons why Conversion idea I had. It involves the Tyranid gargoyles and some skinks. Would their combination of technology and magic allow that, or would they have to stick with blowpipes?
I would say "yes". A big part of the Lizardmen background in the World that Was is that they were created by a technologically advanced race who just up and left one day, abandoning all their technological wonders and leaving it all in the hands of the questionably-capable hands of their Lizardmen servants. It's popped up fairly frequently in various campaigns, armybooks and expansions over the years. Back when Amazons (or Amaxons? Can't remember if they butchered the spelling) were a thing some of them wielded staffs that shot a beam of red light that had effects suspiciously similar to a Lasgun. There's a magic weapon from one of the old Lizardmen books that's described in a similar way, that fires beams of light when an activation glyph on the haft is depressed. That sounds a lot like an "ON" button or trigger to me. The events of the End Times hinted at the degree of tech they had stashed away. Under one temple the Skaven uncovered what appeared to be a computer and a communications device that was receiving messages from what's hinted to be the Eldar. Of course you've also got things like the Solar Engine on the Bastilodon, and Kroq-Gar's fireball shooting glove. The lines between "magic" and "Technology" are very blurred. Then their Temples lifted off the ground and flew into space, so there's that too... From an AoS perspective the Seraphon are conjured by Slann based off their memories of their old Lizardmen servants. The specifics are pretty vague, which I personally find frustrating but leaves it pretty open ended to play around with, but from the Sunblood entry we know that it's possible for the Slann to create an entirely new or evolved version of what they remember. So this implies that it's malleable - they can willingly change what they conjure to be different from the original. Given the very magical nature of the Seraphon I think a lot is possible. Hell, Skinks shooting lightning from their eyes seems feasible to me, so having some kind of laser weapons reminiscent of their Old One masters, or even developing their own Tyranid style bio-tech looking wargear (Maybe the Slann can't be bothered fully forming this new weapons so just kinda half-arsedly melds it to their hands?) seems fine to me. Just my 2c - your mileage may vary.
So my winged skinks with laser guns make sense. I should probably make the guns look a bit more Seraphon-ish.
Glue some snakes and glyphs to it and you'll be fine. Just like steampunk, just glue some cogs and goggles to it. Instant steampunk
@Bowser maybe a crystal too. I should probably write some lore for them. I already have the name of a new Old One. (Ihuicatl. Old One of the sky.)
The tech of the Old ones was a mix if technology and magic that is still undecipherable in the time of Warhammer 40k. While the Lizardmen have many powerful artifacts that the old ones left behind, they never left behind directions on how their technology worked exactly or how to make more. Ps. This may explain their stinginess with their gear, and the loss of even one artifact could be devastating, as it may have been intended to turn the tide in some future battle.
For a good idea of what Seraphon/Old Ones technology looks like go to the GW webstore and have a good look at the Oldblood on Carnosaur, Bastilodon and Skink Starseer. I think those models in particular hint at some underlying but hidden technological wonders, but dont put it front and centre. The Oldblood's gauntlet is obviously a robot hand, but his spear also has a high-tech feel to it and wouldn't look out of place in a 40k army on a Necron lord, or as a power weapon for a Deathwatch veteran. The Bastilodon with Solar Engine is a bit glitzy but uses the "crystal technology" trope common in advanced species in fiction. Most interestingly though is the activation panel at the back - a 5 fingered hand scanner type thing with some rods/wires coming off it. For the Starseer there's a lot going on. His staff is some kind of gadget with moving parts, and a design that's very different to the typical Lizardmen blocky aesthetic. The back of his throne has an arcane device that looks like a fancy clock, possibly to track the movement of celestial bodies. That crystal orb might not be a magic future-seeing orb at all, but a display and input device for an advanced computer hidden inside the throne itself. Then you have the hovery bits underneath. All cool "hidden" technology. But if you want to go a totally different route I don't see anything wrong with that. I've heard of people replacing the weapons on their skinks with pistols from various 40k ranges, giving Saurus warriors chainswords or other such things. There's a lot of conversions out there (some on this very forum if I recall correctly) that use Tyranids as a base as well. Like I said earlier, Seraphon are malleable - they can be changed by the will of their Slann, either willingly or maybe unwillingly. Maybe your Slann got a bump on the head and doesn't remember things quite as they were, or maybe it didn't take millennia of isolation in space quite as well as its colleagues. Maybe your Slann encountered the Tyranids at some point in their space travel, suffered some terrible side effects from the Shadow in the Warp and now the memories of those horrors have infiltrated his every thought, making the Seraphon he conjures forth some horrifying blend of Tyranid and Lizardman. Dammit now I'm getting ideas. Bad Dan!
"Tyranid Goggles and Skinks" was what I got on the first reading... But, sure why not? I think my Slann(s) would imagine it where the Nid pieces were something the Skink could drop from either of its unmutated hands. Blowpipes could be UP-teched by adding a tiny gem to the tip. Micro version of Bastilodon's weapon.