I know it would take time, but if you simply re-worked the game mechanics using the same art to make a new game in the series there might be no additional major costs. I say major costs, because even if you have no art costs, there is still the time that the team puts into the game.
Looking at all the other Arcen titles, Valley seems to have the most active forum people apart from the current title. SH has fallen off and i see no new posts recently. SC as well, which is a little depressing, because that is an awesome, though crazily hard game for me.
BD is doing well with posts, even if you say sales were low, perhaps there is still a chance here, although maybe slower. Posts for LF look dependent on dev posting new information, but am hoping it will pick up and do extremely well, fingers crossed.
(i was really surprised, but happy to see that Valley 1 had 70% approval over Valley 2 had 30 percent.
Anyway, just thought a rework of what you have for the Valley, not getting new art or major costs might produce a new interesting take on the games.
With that said, perhaps reworks of SH, Tidalis, SH and SC with no new art but reworked mechanics (looking for a new type of play, or a different approach to the game, etc...) could draw new attention and sales for those titles, as series, instead of just losses in their current form.
I know it would take your time, especially away from new projects, but it could be done over time, if you feel you have a new approach on several of the old games that could help to make new sales.
Just a thought, re-using assets on hand is not a bad idea, if it leads to something new and interesting and saleable too.