I mean, the SRM is basically already a HEAT payload if you aren't using inferno fuel. The major difference that tandems have is that there are two charges.
The LRM is (apparently) smaller version of a SRM which trades the payload for fuel.
Extra missile types would he neat to have cannonically though, I agree. The issue is that they'd be mostly ineffective in the CBT ruleset if we're going by the Universal Ordnance table, since the largest man portable payload is a class C explodey thing. Unless we get into small scale artillery.
If you go by RPG rules, you can give infantry NARC weapons if you wanted to as well. It just doubles the price of the launcher and the payloads which are supposed to be guided to it.