Battletech

On the modding side there are differing opinions on how much to change. I stuck with mostly the QoL and performance-improvement stuff, but there are mods to do things like add additional mission variety and improve AI. I'm hesitant to provide many recommendations for specific mods because the last time I was playing was before the most recent couple of DLCs, and I don't know if the mods I used back then are still current / updated. An example that I think still works fine would be QOL Upgrades 2 - Thermals and Headlights - small things like headlights imo should just be in the base game but aren't.

DLC-wise I've only played half of them (waiting on sales for the rest!), but for example Legend of the Kestrel Lancers gives you a self-contained storyline of maybe a dozen missions or so. The main thing about those missions for me personally is that the maps are unique (and not just rehashed procgen) and there's supporting dialogue with some cinematics etc. So the gameplay overall is mostly pretty similar if you drill down to it, but the window dressing on top of it largely addressed the lifeless feeling of all the procgen missions (again, at least for me!).

It looks like both The Dragon's Gambit and Rise of Rasalhague are similar in that they add a set of new once-off missions, but I've yet to play them personally. I guess you could try watching a bit of gameplay from a let's play of them or something to see if it tickles your fancy. The reviews on the DLCs hovers around 70% on Steam, so it's not as though they were universally well-liked and to be honest they might not be enough for you if you weren't at least lukewarm on the base game already.