Thank you so much for trying to lead the conversation away from glalie.mega Steelix is almost complete trash...but it still has niches.
The most obvious is that pair it in doubles with Hippo/TTar and then spam EQ. Although since this is OU, I guess that won't apply.
Honestly I don't think many people will use mega Steelix, as all the OP megas *cough* mence *cough* will be used until they are banned...
But I guess the best thing you can do with it so that it actually has a niche over mega Aggron is to run a Sand team supporter set. Somehow steelix doesn't get TWave and its ability is obviously worse than mega Aggron's without sand, so you need to find what you can do with it. I guess Steelix could be a bulky sand setter on sand teams alongside TTar, but with TTar+Exca+Steelix you're just asking to get run over by Conk not to mention a ton of weaknesses you stack. Still, Steelix can run something like SR/Sandstorm/Earthquake/Gyro Ball, or opt for something more team-supporting like Roar in the last slot. I personally wouldn't do that though, as you need both STAB moves to hit hard, as Sand Force is going to be what you use to hit harder than stuff like mega Aggron. Also, a bonus is that your pre-mega evo ability is Sturdy, allowing you guaranteed SR/Sandstorm if you really need it.
Lati@s stand out as pretty obvious partners as they can Defog and take on Zard-Y, non Ice Punch mega Swampert, Lando-I, Rotom-W and Keldeo, who are big problems for Steelix and sand. Meanwhile, mega Steelix trolls stuff like Bisharp, Weavile, fairies(mega Altaria), Scizor and mega Beedrill.
I still really can't see mega Steelix making the cut for OU tho...too many other good megas out there that give it tough competition
Anyway, what happened to those old megas? I don't believe zard x will be nearly as dominant because of the new base 110s, and pinsir is basically the same as megamence except a bit more powerful and slower and has priority. Some people have been using mega aero as a check to all of these new megas, specifically mence.