Not necessarily. A simple retheme could be hinted at over Fright Nights (to give people a chance to have a final ride), then marketed from early next year. Or even just announced next year.
I agree a complete new ride is extremely unlikely, if for no other reason than the fact planning permission would be a nightmare. Plus the fact there is still the Beach development which could happen.
One would certainly hope there's thoughts/conversations about the next *big* thing for the park. Though I imagine there's a level of dependence on what the situation with Merlin is there. Arguably though, a major investment shouldn't be the next priority, but rather a couple of medium investments to sort out the support line up (Zodiac and Rumba, I'm looking at you)