I programmed the OT features for a script font with a very similar approach.
What I can say from that experience is do not use ligatures, if you avoid them the font you could support easily any character combination and any language you type. Of course you will need to do a good amount of alternative characters to have a good connection in both sides. For instance in the font that I made the OT features the letter /e has 6 alternatives but it depends on the design.
For me the best thing to do was to create an double entry spreadsheet to “discover” the quantity of alternatives needed.
Increasing the glyph number sound like a lot more work to do but with some scripts you can automate the creation of accented characters, ot classes, etc.