Plans change all the time so you can never really rely on what's happened in the past, or what rumours are going around. Especially at Towers, plans can change overnight there.
The theme park is just fine with roller coasters for now I think. The Smiler has rounded off the variety of rides available quite well. A dark ride seems the next logical step for a big investment. 2015 will almost certainly be a low to medium investment year for the theme park. The lodges will not affect the park investments at all.
Alton Towers is a weird one as they don't follow patterns like other parks do. Towers is a big park and has a big draw of guests. So it's the only park that Merlin really do pump in millions year on year. It really wouldn't surprise me if we saw two big investment years, a dark ride in 2016, SW8 in 2017. But then as I say plans can change overnight with them so god-knows what we will see in reality over the coming years.
You've got to give it to Merlin, despite their flaws, in terms of investments they have spoiled Towers since 2008.