Given that we've talked a lot about BTDB2, there could be other bloons sequels that we are interested in. Here are some ideas I thought of:
Regen bloons: give blops but don't add to total pop count if regrown. Can be farmed?
New bloons: brick, purple (immunities of glass and ceramic), maybe others?
New blimps: HTA, BRC, DDT (lead and black but not camo, maybe phasing?), BAD (perhaps a later zone boss)
Lead becomes a property?
Higher tier upgrades?