Maybe something like what the Necrarch list can do in modifying Abyssal Terrors, where they have a "Tough Hide" save that costs 5 points for 6+, then doubles in cost for each additional +1? Though, in this case I'd drop it to 1 point initially. That way it would look like:
6+ Save: 1 pt/model
5+ Save: 2 pts/model
4+ Save: 4 pts/model
3+ Save: 8 pts/model
2+ Save: 16 pts/model
Though in theory, we could incorporate that into the Merman idea. Start with something cheap like a Zombie, then allow various upgrades that become increasingly expensive the more options are taken?
1 Upgrade: 1 pt/model
2 Upgrades: 3 pts/model
3 Upgrades: 6 pts/model
4 Upgrades: 10 pts/model
And so on, in that fashion. That would allow the construction of strong defensive infantry, or strong offensive infantry, or balanced infantry that are effective but not strong in either category, depending on how many points one was prepared to sink into the unit.