I believe that if SBS Transit and SMRT merges, it may be forced to compromise and use both double-deckers and bendies alongside standard buses. This is not an entirely bad thing though - I have noticed high-load bus routes on either side being forced to use standard buses because of this or that restriction, and that they don't have the type of bus necessary to get around it (for example, the height restrictions along route 105, and the requirement to u-turn on the old route 804 before it was amended via a new road). In fact, I have seen a photo of SBS 999(U?) on route 105 somewhere before, but I'm not sure if it's one of those posed photographs.
And, besides seeing what can only happen now in our fantasy (imagine: Volvo B10Ms on 851, Dennis Lances on 133, Volvo B9TLs on 961, Mercedes Habits on route 18

), I can see that a lot of bus routes will be rationalised, akin to what happened in the 1970's when the three bus companies became one under then-SBS. Most likely, sets of somewhat-related routes operated by the two companies (like routes 27/34/39/85/89/109/161/168/858/965/969 - all of which are north - north-east - east routes to some extent) will get very major changes, or might have some of their routes even withdrawn.
The reality, though, is that an SBS Transit-SMRT merger looks unlikely for now. SMRT's predecessor, TIBS, was set up to provide some friendly competition originally, and commuters, while demanding for more integrated services, have also been calling for more competition. Even if both companies were to merge, it could very well happen under a plan to liberalise the public bus sector in Singapore, so the resulting situation could be one large company (SBS Transit + SMRT) dominating the public bus sector alongside with smaller companies competing with it.