Personally, I think 2 heavies at 5 might do it. Centurions are tougher with better guns for walking, assault terminators are better at everything else. With only 1 heavy, they can't alpha strike well (Chaos gets combimeltas, which is a game changer in that regard), with only a 5++ at T4 with 1 wound, they whittle too quickly to play the attrition game, and if in a transport, they might as well be assault terminators!
I've used them to success with teleport homers scattered everywhere as a tactical game changer, but honestly, assault terminators fill that role better. Storm bolters just aren't enough firepower. Back in the early days of 3rd, when moving meant you only shot once at 12", the storm bolter had double the firepower of a tactical marine on the move at 12" or double the power of a stationary tac marine at 12"-24". Then they had some utility. With the changes to rapid fire, the storm bolter isn't bringing you anything, which makes you reliant on the heavy to do the lifting (as Tacs rely on their special and/or heavy) and the limitation to 1 just isn't enough. Tacs can take meltas or plasma, with a second combi on a sarge. Terminators can take an assault cannon or a missile launcher, both costing way more, neither reliable against 2+ or armor (Statistically assault cannons are way better over time, but one round of shooting, give me a meltagun or plasmagun), and the entrapment into a powersword on the sarge is limiting. A maul would be excellent, a thunderhammer better (I miss the days of my power sword/ thunderhammer terminator assault sarges).
On topic:
Surprised that DA and CSM are being leapfrogged. Eldar could be another supplement; say Biel-Tann or Sam Hainn; Admech makes me want to sQUEEE!