Mudskippers? There will never be enough space to make out a paludarium out of it just for their convenience. They're an absolute nono for that size of tank.
Not all bumblebee gobies are brackish but with the tricky identification issues, almost all BBGs will still thrive in brackish water regardless or even hard alkaline water.
The other suggestions correctly given are bettas, pea puffers, cardinal tetras and guppies. I'd personally give bettas, small colorful tetras (research first which species you want because there are some that need plenty of space), and fancy guppies. As a word of warning however, a lot of fancy guppies especially those intended for shows are very sensitive so be careful what you choose.