The "track tank with highest heat" idea would be brilliant IRL. That's how your regular Stinger missiles work. In Empires though, I would dislike my missile suddenly fly offscreen and hit the hottest tank of the opposing team, which can be somewhere across the map, and not where I want to do damage. Having a weapon that can stall heavy tanks lategame is (at least for me) a good mechanic, seeing as you aren't directly damaging it, but taking it out of action. A tank that isn't moving is a dead tank, and having your 3 heavies stalled and walled/stickied/mined would be something horrible. With a BioRPG grenadiers can still be very effective vs heavies and mediums, even providing some real good anti-CV weapons.
The way you describe the Plasma RPG, its more like a flying, impact based sticky stun, which would be entirely overpowered. A heat seeker should only target things in a small cone (45-60 degrees), and possibly have an upgrade to increase the target cone, making lower accuracy required. A grenadier Bio-Rocket (which should be unguided entirely due to bio's abillity to keep damaging armor over time) would be useful, but should have to be researched. Anyway, a late game stall-causing Plasma RPG would only be useful for the attacking team due to the fact that the defending team won't be able to produce enough tanks, and because the atackers will likely strike en masse, meaning one or 2 disabled tanks won't be that detrimental, but the defending team's only tank(s) being disabled is sure death. Another thing to keep in mind is that it is unlikely that a single grenadier will be able to disable more than one tank before being killed by another. As an ambush tactic, a few grens with riflemen armed with stickies would be effective on maps with chokepoints, but that takes cooperation that is rare in a normal, public game. Also, the damage the RPG would do on it's own would have to be minimal due to its high heat to target.
