What I would do is create experimental formations that are independent of a particular list (print them at the end of the compendium). Experimental formations would never be tournament legal, but should be suitable for friendly games. So if we stat up the Spartan, we should also create a more-or-less standard Spartan formation, note that it is an experimental formation for use in friendly games in lists that don't have access to it, then forget about it until someone uses it in a list. Avoids too much meddling with fairly stable lists, and still lets people take new shiny toys with established lists.
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Er...Assault Terminators and Scout Bikes both existed when the current list was created.

Plus it means these formations get some kind of playtesting before thier inclusion into a list

Testing removed from the rest of a list's context might not be all that useful.

How is it removed from the rest of a list's context?

Context is one of the most important considerations in Epic list development, and it's why a plug-on modular approach won't be all that useful.

Context matters. For example, an infantry unit with 2x CC2+ attacks added to the Codex Space Marine list would be worth a lot, because you could Thunderhawk it straight into CC.

The same unit added to a variant list without air transports would be worth less, because it would rarely make it into CC.

Context is one of the most important considerations in Epic list development, and it's why a plug-on modular approach won't be all that useful.

*shrug*

I have a fair bit of experience getting lists balanced and finished ; I don't think "modular" development will be all that useful in generating balanced army lists. From my experience, I can only say that such an approach would have slowed down the developments I've been involved in, not increased the pace.

As does calling for the addittion of every new unit GW dreams up to lists that are already finished.

Shame the game is tied to the 40k IP in some ways, given that lately GW seem to be doing their utmost to retconn the background and introduce ever more units.