Great little game - really good seeing a board game style thing in the jam. Art style and jam limitation actually really suits it and gives it a lovely charm. It was a bit easy, but this would fit with a tutorial type level. Improvements to enemy AI or maybe different piece types (maybe you can include a dragon who can has a different movement pattern or can attack without surrounding pieces) at higher levels could give variation. You could have a story/campaign mode... Really enjoyed it - also a multiplayer option would be good!
I made conscious decision for my first game jam entry to try and not overcomplicate it, and nick the gameplay more or less 1:1 from existing public domain game, and save some time having to play-test it.
Also, 7x7 grid with one line border perfectly fit in 64x64 pixel grid. The more advanced Tafl game has 11x11 grid, so without lowrez requirements, might make it a bit easier, not that good at actual graphic design I'm afraid.
New pieces and progressing through levels is good idea, could make it rouge-like, every next level you only start with reminder of warriors, so you could either rush through quickly, to sacrifice vikings, or carefully manoeuvre to keep the band safe, but have more pieces on later levels.
Hot-seat MP shouldn't be too hard to implement.