Getting away from the issue of bigger battles (and I agree that the issues of extended mission time and focused fire are showstoppers, as any random mission that essentially guarantees you will lose multiple mechs is not going to be one people are going to want to play in either campaign or career mode, nor are they generally going to want to play hour+ missions outside of flashpoints), I expect we have at least one more year worth of DLCs (probably more) before they begin seriously thinking about a Battletech 2.
HBS is the developers, but much of the marketing/distribution is being handled by Paradox. When to move on to create a sequel is at least partially a marketing decision, and while I can't imagine Paradox would overrule HBS if they said "ok, now let's make Battletech II," I expect Paradox's general policy of "sell the base game and continue adding on to it for as much as possible before" will likely inform some of their decisions. CK2 (the first Paradox game to use the current DLC policy) was released in February 2012, and is still being supported (the last major patch was last month, and the most recent paid DLC was this past November, i.e. 6.5 years after the first release). Likewise the other major Paradox titles of this generation are currently going strong years after their release. That clearly seems to be a strategy that is more or less working for them (and you can trace a lot of Paradox's rise from niche to significant market player to the period from the release of CK2), and one I imagine they'd recommend to HBS.
I doubt we'll see Battletech development last as long as CK2 has (if nothing else, I imagine at some point people will want Clans, and that very much should be its own game), but I expect at least another year or two before they start working on Battletech 2. It took ~3 years from announcement to release of the current game; making a sequel will likely take somewhat less time (no kickstarter, for one thing, as they would presumably have funding from Paradox), but still long enough that I imagine they will want to milk as much out of the current game as possible before switching over to developing a sequel.
The current game is a solid base with plenty of things left for them to expand on. I doubt we see fundamental changes to gameplay, but I could easily imagine things like introducing infantry to shake things up. Even if we just look at new mechs, there are what, 13? 3025 PGI mech models left to adapt; some of them will likely be added in Heavy Metal, but I doubt all of them will (and of course, there are plenty of other mechs out there if HBS wants to try their hand at making more mechs a la the Hatchetman, not to mention vehicles). Not to mention flashpoints with every expansion (even if not the main focus, I expect we will see more flashpoints with each expansion, if only because they are "easy," let you show off new mechanics (look at the UW flashpoints, pretty much all of which are focused on showing off either the new mission types or the new mechs) and give that part of the team something to do during the development of the DLC (if someone is an excellent scenario designer but doesn't know their way around the modeling software, you want them writing excellent scenarios instead of causing problems for the art team, and vice versa).