Nuke is ridiculous. I'll offer some alternatives though:
Suitcase EMP: Call Merryweather and they'll set an EMP device off in the area of your choice (500ft radius in all directions maybe?), rendering all vehicles useless in the vicinity, as well as knocking out phones, sticky bombs, mini maps and anything else electrical. Would be good for giving a little grief back to those annoying tank and Buzzard folks. Lasts for 5 mins and you can't call another for a long while so people don't end up EMPing the whole city constantly.
Laser designator for artillery/airstrikes/drone strike: This one's been mentioned already but it's a good idea and I'll try to expand. You'd have to be within a block or two radius to stop you crazy kids camping on Maze Bank Tower painting everyone with a sniper rifle. Once painted, either drop an artillery barrage, home in with an airstrike, or have a drone target them. The target would get a warning when painted similar to a bounty warning so they have chance to get cover. In the case of a drone strike, maybe have a hacking mini game where you have a chance to turn the drone back on it's original user before it gets you.
Shape charge / Anti Tank mines: Something to level the playing field a little, shape charge to one hit kill them or drop a mine and get the f*cker to chase you over it. Would give tank drivers a little pause instead of just rolling around like they own the place like they do now.