As suggested on IRC it would be interesting to have a new tank type exhibit properties based on the material within the tank.

Example:

Water - the tank would put out entities that stand on it (would need to take the nether into account), turn lava source blocks it touches into obsidian, etc.

Lava - the tank would ignite entities that stand on it (or touch it). This same property could also be leveraged with the new heat feature of liquids in forge... assuming mods assign it.

Mod added liquids:

Liquid Ender - the tank could randomly teleport a player that touches it a short distance

Liquid redstone - the tank could emit a redstone signal based on the percentage of the tank full. Incredibly useful - analogous to a mercury switch.

Liquid glowstone - the tank could give any entity that touches it a jump boost.

And so on. It offers a lot of possibilities and flexibility.