As Only War matures as a game, I can definitely see more las-variants showing up in the armouries. It IS FFG after all. The precedent has already been set. The Inquisitor's Handbook, Into the Storm, Rites of Battle, the Tome of (insert Chaos association here)…. If it is one thing FFG has never let us down on is new weapons and gear. I've got faith Hammer of the Emperor will be the same.
As for creating stats for the fluff guns in the interim, it shouldn't be that hard.
The venerable Lucius 98 of the Death Korps would be a snap. More damage, much less ammo (fluff says like 25 shots before reload) and no rapid fire - single shot only.
The Mars Pattern that the Tallarns use looks to be a long rifle. If we take the design into account, as well as the history and theme of the regiment, we're likely looking at something not unlike a las-lock. Single shot, more than likely - it looks a little too…pretty…to have the internal mechanics for cyclical firing, purely from a design stand-point. The Desert Raiders are supposed to be hit-and-fade, so the gun prolly has wicked range and accuracy.
The Voss pattern used by the Armageddon Steel Legions looks just like a cut down rifle, smaller so it can be manoevered safely within the confines of a Chimera, as they are mechanised infantry and all. Maybe cut down the range, like a las-carbine but otherwise, it's a standard lasgun?
As for Ryza-pattern guns? Maybe a penalty to certain skills? -30 to Intimidate? I mean, really? I just couldn't take a trooper seriously, carrying what looks like an ugly left-over from a really low budget Sci-Fi show. Or maybe a giant cake decorator? I mean, seriously. Yech. Technical Design - CRITICAL FAIL. Just my 2 cents.