There's a dirty theory going around some forums that ME3 will be just Shepard, like ME2's last DLC was.
That would be pretty stupid.
ME2´s party combat system may be nothing to write home about, but (at least on higher difficulties) party composition and abilities add quite a substantial layer in regards to tactics and play style and, more importantly, it´s the overall feel of party participation that´s really important (and staple for the series).
Few Shepard-only combat scenarios would be good though - when done appropriately, this type of challenge tends to be interesting in otherwise party-based games, having to rely solely on the main char´s abilities and all.
My "dirty" theory is that in ME3 there´ll be about 3 non-optional new companions and some old ones will either have cameos or return as optional squad members, depending on save import, with the possibility to influence how some quests will play out. For a non-imported Shepard, implementing some default alive/dead state for these shouldn´t be a problem.
Bioware didn´t have problems implementing this kind of optional content in the past (DA:O, for example) and hopefully doesn´t have now.
Of course DA2 and Arrival DLC put some doubts on how ME3 will turn out.
